Streaming engine with deferred exception reporting

ABSTRACT

This invention is a streaming engine employed in a digital signal processor. A fixed data stream sequence is specified by a control register. The streaming engine fetches stream data ahead of use by a central processing unit and stores it in a stream buffer. Upon occurrence of a fault reading data from memory, the streaming engine identifies the data element triggering the fault preferably storing this address in a fault address register. The streaming engine defers signaling the fault to the central processing unit until this data element is used as an operand. If the data element is never used by the central processing unit, the streaming engine never signals the fault. The streaming engine preferably stores data identifying the fault in a fault source register. The fault address register and the fault source register are preferably extended control registers accessible only via a debugger.

RELATED APPLICATIONS

This application patent application is an improvement over U.S. patentapplication Ser. No. 14/331,986 filed Jul. 15, 2014 entitled HIGHLYINTEGRATED SCALABLE, FLEXIBLE DSP MEGAMODULE ARCHITECTURE which claimspriority from U.S. Provisional Patent Application Ser. No. 61/846,148filed Jul. 15, 2013.

TECHNICAL FIELD OF THE INVENTION

The technical field of this invention is digital data processing andmore specifically control of streaming engine used for operand fetching.

BACKGROUND OF THE INVENTION

Modern digital signal processors (DSP) faces multiple challenges.Workloads continue to increase, requiring increasing bandwidth. Systemson a chip (SOC) continue to grow in size and complexity. Memory systemlatency severely impacts certain classes of algorithms. As transistorsget smaller, memories and registers become less reliable. As softwarestacks get larger, the number of potential interactions and errorsbecomes larger.

Memory bandwidth and scheduling are a problem for digital signalprocessors operating on real-time data. Digital signal processorsoperating on real-time data typically receive an input data stream,perform a filter function on the data stream (such as encoding ordecoding) and output a transformed data stream. The system is calledreal-time because the application fails if the transformed data streamis not available for output when scheduled. Typical video encodingrequires a predictable but non-sequential input data pattern. Often thecorresponding memory accesses are difficult to achieve within availableaddress generation and memory access resources. A typical applicationrequires memory access to load data registers in a data register fileand then supply to functional units which preform the data processing.

SUMMARY OF THE INVENTION

This invention is a streaming engine employed in a digital signalprocessor. A fixed data stream sequence is specified by storingcorresponding parameters in a control register. The data stream includesplural nested loops. Once started the data stream is read only andcannot be written. This generally corresponds to the needs of areal-time filtering operation.

The streaming engine includes an address generator which producesaddress of data elements and a steam head register which stores dataelements next to be supplied to functional units for use as operands.The streaming engine fetches stream data ahead of use by the centralprocessing unit core in a stream buffer constructed like a cache. Thestream buffer cache includes plural cache lines, each including tagbits, at least one valid bit and data bits. Upon occurrence of a faultreading data from the memory, streaming engine identifies the dataelement triggering the fault preferably storing this address in a faultaddress register. The streaming engine defers signaling this fault tothe central processing unit until this data element is used as anoperand. If the data element is never used by the central processingunit, the streaming engine never signals the fault. The streaming enginepreferably stores data identifying the fault in a fault source register.The fault address register and the fault source register are preferablyextended control registers accessible only via a debugger.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of this invention are illustrated in thedrawings, in which:

FIG. 1 illustrates a dual scalar/vector datapath processor according toone embodiment of this invention;

FIG. 2 illustrates the registers and functional units in the dualscalar/vector datapath processor illustrated in FIG. 1;

FIG. 3 illustrates a global scalar register file;

FIG. 4 illustrates a local scalar register file shared by arithmeticfunctional units;

FIG. 5 illustrates a local scalar register file shared by the multiplyfunctional units;

FIG. 6 illustrates a local scalar register file of shared by theload/store units;

FIG. 7 illustrates a global vector register file;

FIG. 8 illustrates a predicate register file;

FIG. 9 illustrates a local vector register file shared by arithmeticfunctional units;

FIG. 10 illustrates a local vector register file shared by the multiplyand correlation functional units;

FIG. 11 illustrates pipeline phases of a central processing unitaccording to a preferred embodiment of this invention;

FIG. 12 illustrates sixteen instructions of a single fetch packet;

FIG. 13 illustrates an example of the instruction coding of instructionsused by this invention;

FIG. 14 illustrates the bit coding of a condition code extension slot 0;

FIG. 15 illustrates the bit coding of a condition code extension slot 1;

FIG. 16 illustrates the bit coding of a constant extension slot 0;

FIG. 17 is a partial block diagram illustrating constant extension;

FIG. 18 illustrates the carry control for SIMD operations according tothis invention;

FIG. 19 illustrates a conceptual view of the streaming engines of thisinvention;

FIG. 20 illustrates a first example of lane allocation in a vector;

FIG. 21 illustrates a second example of lane allocation in a vector;

FIG. 22 illustrates a basic two dimensional stream;

FIG. 23 illustrates the order of elements within the example stream ofFIG. 21;

FIG. 24 illustrates extracting a smaller rectangle from a largerrectangle;

FIG. 25 illustrates how the streaming engine would fetch the stream ofthis example with a transposition granularity of 4 bytes;

FIG. 26 illustrates how the streaming engine would fetch the stream ofthis example with a transposition granularity of 8 bytes;

FIG. 27 illustrates the details of streaming engine of this invention;

FIG. 28 illustrates a stream template register of this invention;

FIG. 29 illustrates sub-field definitions of the flags field of thestream template register of this invention;

FIG. 30 illustrates a partial schematic diagram showing the streamingengine supply of data of this invention;

FIG. 31 illustrates a conceptual view of an embodiment of storing thedata stream;

FIG. 32 illustrates the details of the cache construction of the databuffer in the streaming engine (Prior Art);

FIG. 33 is a partial schematic diagram illustrating generation of theearly address and early loop count for loop 0;

FIGS. 34A and 34B together are a flow chart illustrating the operationof the early address generator;

FIG. 35 is a partial schematic diagram illustrating generation of thelate address and late loop count for loop 0;

FIG. 36 is a flow chart illustrating the operation of the late addressgenerator;

FIG. 37 illustrates an exemplary bit assignment of an extended controlregister storing early and late loop counts;

FIG. 38 illustrates an exemplary bit assignment of an extended controlregister storing early and late addresses;

FIG. 39 illustrates an exemplary bit assignment of an extended controlregister storing the virtual address of a fetch generating an error inthe streaming engine;

FIG. 40 illustrates an exemplary bit assignment of an extended controlregister storing data detailing the nature of a detected error;

FIG. 41 illustrates the error detection and correction of thisinvention;

FIG. 42 is a schematic diagram illustrating a multiprocessor,multi-memory system to which the fetch hysteresis of this invention isapplicable;

FIG. 43 is a partial schematic diagram illustrating hardware of anexemplary embodiment of this invention including a priority techniqueemploying hysteresis; and

FIG. 44 is a flow chart illustrating a priority technique employinghysteresis.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 illustrates a dual scalar/vector datapath processor according toa preferred embodiment of this invention. Processor 100 includesseparate level one instruction cache (L1I) 121 and level one data cache(L1D) 123. Processor 100 includes a level two combined instruction/datacache (L2) 130 that holds both instructions and data. FIG. 1 illustratesconnection between level one instruction cache 121 and level twocombined instruction/data cache 130 (bus 142). FIG. 1 illustratesconnection between level one data cache 123 and level two combinedinstruction/data cache 130 (bus 145). In the preferred embodiment ofprocessor 100 level two combined instruction/data cache 130 stores bothinstructions to back up level one instruction cache 121 and data to backup level one data cache 123. In the preferred embodiment level twocombined instruction/data cache 130 is further connected to higher levelcache and/or main memory in a manner not illustrated in FIG. 1. In thepreferred embodiment central processing unit core 110, level oneinstruction cache 121, level one data cache 123 and level two combinedinstruction/data cache 130 are formed on a single integrated circuit.This signal integrated circuit optionally includes other circuits.

Central processing unit core 110 fetches instructions from level oneinstruction cache 121 as controlled by instruction fetch unit 111.Instruction fetch unit 111 determines the next instructions to beexecuted and recalls a fetch packet sized set of such instructions. Thenature and size of fetch packets are further detailed below. As known inthe art, instructions are directly fetched from level one instructioncache 121 upon a cache hit (if these instructions are stored in levelone instruction cache 121). Upon a cache miss (the specified instructionfetch packet is not stored in level one instruction cache 121), theseinstructions are sought in level two combined cache 130. In thepreferred embodiment the size of a cache line in level one instructioncache 121 equals the size of a fetch packet. The memory locations ofthese instructions are either a hit in level two combined cache 130 or amiss. A hit is serviced from level two combined cache 130. A miss isserviced from a higher level of cache (not illustrated) or from mainmemory (not illustrated). As is known in the art, the requestedinstruction may be simultaneously supplied to both level one instructioncache 121 and central processing unit core 110 to speed use.

In the preferred embodiment of this invention, central processing unitcore 110 includes plural functional units to perform instructionspecified data processing tasks. Instruction dispatch unit 112determines the target functional unit of each fetched instruction. Inthe preferred embodiment central processing unit 110 operates as a verylong instruction word (VLIW) processor capable of operating on pluralinstructions in corresponding functional units simultaneously.Preferably a complier organizes instructions in execute packets that areexecuted together. Instruction dispatch unit 112 directs eachinstruction to its target functional unit. The functional unit assignedto an instruction is completely specified by the instruction produced bya compiler. The hardware of central processing unit core 110 has no partin this functional unit assignment. In the preferred embodimentinstruction dispatch unit 112 may operate on plural instructions inparallel. The number of such parallel instructions is set by the size ofthe execute packet. This will be further detailed below.

One part of the dispatch task of instruction dispatch unit 112 isdetermining whether the instruction is to execute on a functional unitin scalar datapath side A 115 or vector datapath side B 116. Aninstruction bit within each instruction called the s bit determineswhich datapath the instruction controls. This will be further detailedbelow.

Instruction decode unit 113 decodes each instruction in a currentexecute packet. Decoding includes identification of the functional unitperforming the instruction, identification of registers used to supplydata for the corresponding data processing operation from among possibleregister files and identification of the register destination of theresults of the corresponding data processing operation. As furtherexplained below, instructions may include a constant field in place ofone register number operand field. The result of this decoding issignals for control of the target functional unit to perform the dataprocessing operation specified by the corresponding instruction on thespecified data.

Central processing unit core 110 includes control registers 114. Controlregisters 114 store information for control of the functional units inscalar datapath side A 115 and vector datapath side B 116 in a mannernot relevant to this invention. This information could be modeinformation or the like.

The decoded instructions from instruction decode 113 and informationstored in control registers 114 are supplied to scalar datapath side A115 and vector datapath side B 116. As a result functional units withinscalar datapath side A 115 and vector datapath side B 116 performinstruction specified data processing operations upon instructionspecified data and store the results in an instruction specified dataregister or registers. Each of scalar datapath side A 115 and vectordatapath side B 116 includes plural functional units that preferablyoperate in parallel. These will be further detailed below in conjunctionwith FIG. 2. There is a datapath 117 between scalar datapath side A 115and vector datapath side B 116 permitting data exchange.

Central processing unit core 110 includes further non-instruction basedmodules. Emulation unit 118 permits determination of the machine stateof central processing unit core 110 in response to instructions. Thiscapability will typically be employed for algorithmic development.Interrupts/exceptions unit 119 enable central processing unit core 110to be responsive to external, asynchronous events (interrupts) and torespond to attempts to perform improper operations (exceptions).

Central processing unit core 110 includes streaming engine 125.Streaming engine 125 supplies two data streams from predeterminedaddresses typically cached in level two combined cache 130 to registerfiles of vector datapath side B. This provides controlled data movementfrom memory (as cached in level two combined cache 130) directly tofunctional unit operand inputs. This is further detailed below.

FIG. 1 illustrates exemplary data widths of busses between variousparts. Level one instruction cache 121 supplies instructions toinstruction fetch unit 111 via bus 141. Bus 141 is preferably a 512-bitbus. Bus 141 is unidirectional from level one instruction cache 121 tocentral processing unit core 110. Level two combined cache 130 suppliesinstructions to level one instruction cache 121 via bus 142. Bus 142 ispreferably a 512-bit bus. Bus 142 is unidirectional from level twocombined cache 130 to level one instruction cache 121.

Level one data cache 123 exchanges data with register files in scalardatapath side A 115 via bus 143. Bus 143 is preferably a 64-bit bus.Level one data cache 123 exchanges data with register files in vectordatapath side B 116 via bus 144. Bus 144 is preferably a 512-bit bus.Busses 143 and 144 are illustrated as bidirectional supporting bothcentral processing unit core 110 data reads and data writes. Level onedata cache 123 exchanges data with level two combined cache 130 via bus145. Bus 145 is preferably a 512-bit bus. Bus 145 is illustrated asbidirectional supporting cache service for both central processing unitcore 110 data reads and data writes.

Level two combined cache 130 supplies data of a first data stream tostreaming engine 125 via bus 146. Bus 146 is preferably a 512-bit bus.Streaming engine 125 supplies data of this first data stream tofunctional units of vector datapath side B 116 via bus 147. Bus 147 ispreferably a 512-bit bus. Level two combined cache 130 supplies data ofa second data stream to streaming engine 125 via bus 148. Bus 148 ispreferably a 512-bit bus. Streaming engine 125 supplies data of thissecond data stream to functional units of vector datapath side B 116 viabus 149. Bus 149 is preferably a 512-bit bus. Busses 146, 147, 148 and149 are illustrated as unidirectional from level two combined cache 130to streaming engine 125 and to vector datapath side B 116 in accordancewith the preferred embodiment of this invention.

In the preferred embodiment of this invention, both level one data cache123 and level two combined cache 130 may be configured as selectedamounts of cache or directly addressable memory in accordance with U.S.Pat. No. 6,606,686 entitled UNIFIED MEMORY SYSTEM ARCHITECTURE INCLUDINGCACHE AND DIRECTLY ADDRESSABLE STATIC RANDOM ACCESS MEMORY.

FIG. 2 illustrates further details of functional units and registerfiles within scalar datapath side A 115 and vector datapath side B 116.Scalar datapath side A 115 includes global scalar register file 211,L1/S1 local register file 212, M1/N1 local register file 213 and D1/D2local register file 214. Scalar datapath side A 115 includes L1 unit221, S1 unit 222, M1 unit 223, N1 unit 224, D1 unit 225 and D2 unit 237.Vector datapath side B 116 includes global vector register file 231,L2/S2 local register file 232, M2/N2/C local register file 233 andpredicate register file 234. Vector datapath side B 116 includes L2 unit241, S2 unit 242, M2 unit 243, N2 unit 244, C unit 245 and P unit 246.There are limitations upon which functional units may read from or writeto which register files. These will be detailed below.

Scalar datapath side A 115 includes L1 unit 221. L1 unit 221 generallyaccepts two 64-bit operands and produces one 64-bit result. The twooperands are each recalled from an instruction specified register ineither global scalar register file 211 or L1/S1 local register file 212.L1 unit 221 preferably performs the following instruction selectedoperations: 64-bit add/subtract operations; 32-bit min/max operations;8-bit Single Instruction Multiple Data (SIMD) instructions such as sumof absolute value, minimum and maximum determinations; circular min/maxoperations; and various move operations between register files. Theresult may be written into an instruction specified register of globalscalar register file 211, L1/S1 local register file 212, M1/N1 localregister file 213 or D1/D2 local register file 214.

Scalar datapath side A 115 includes S1 unit 222. S1 unit 222 generallyaccepts two 64-bit operands and produces one 64-bit result. The twooperands are each recalled from an instruction specified register ineither global scalar register file 211 or L1/S1 local register file 212.S1 unit 222 preferably performs the same type operations as L1 unit 221.There optionally may be slight variations between the data processingoperations supported by L1 unit 221 and S1 unit 222. The result may bewritten into an instruction specified register of global scalar registerfile 211, L1/S1 local register file 212, M1/N1 local register file 213or D1/D2 local register file 214.

Scalar datapath side A 115 includes M1 unit 223. M1 unit 223 generallyaccepts two 64-bit operands and produces one 64-bit result. The twooperands are each recalled from an instruction specified register ineither global scalar register file 211 or M1/N1 local register file 213.M1 unit 223 preferably performs the following instruction selectedoperations: 8-bit multiply operations; complex dot product operations;32-bit bit count operations; complex conjugate multiply operations; andbit-wise Logical Operations, moves, adds and subtracts. The result maybe written into an instruction specified register of global scalarregister file 211, L1/S1 local register file 212, M1/N1 local registerfile 213 or D1/D2 local register file 214.

Scalar datapath side A 115 includes N1 unit 224. N1 unit 224 generallyaccepts two 64-bit operands and produces one 64-bit result. The twooperands are each recalled from an instruction specified register ineither global scalar register file 211 or M1/N1 local register file 213.N1 unit 224 preferably performs the same type operations as M1 unit 223.There may be certain double operations (called dual issued instructions)that employ both the M1 unit 223 and the N1 unit 224 together. Theresult may be written into an instruction specified register of globalscalar register file 211, L1/S1 local register file 212, M1/N1 localregister file 213 or D1/D2 local register file 214.

Scalar datapath side A 115 includes D1 unit 225 and D2 unit 237. D1 unit225 and D2 unit 237 generally each accept two 64-bit operands and eachproduce one 64-bit result. D1 unit 225 and D2 unit 237 generally performaddress calculations and corresponding load and store operations. D1unit 225 is used for scalar loads and stores of 64 bits. D2 unit 237 isused for vector loads and stores of 512 bits. D1 unit 225 and D2 unit237 preferably also perform: swapping, pack and unpack on the load andstore data; 64-bit SIMD arithmetic operations; and 64-bit bit-wiselogical operations. D1/D2 local register file 214 will generally storebase and offset addresses used in address calculations for thecorresponding loads and stores. The two operands are each recalled froman instruction specified register in either global scalar register file211 or D1/D2 local register file 214. The calculated result may bewritten into an instruction specified register of global scalar registerfile 211, L1/S1 local register file 212, M1/N1 local register file 213or D1/D2 local register file 214.

Vector datapath side B 116 includes L2 unit 241. L2 unit 221 generallyaccepts two 512-bit operands and produces one 512-bit result. The twooperands are each recalled from an instruction specified register ineither global vector register file 231, L2/S2 local register file 232 orpredicate register file 234. L2 unit 241 preferably performs instructionsimilar to L1 unit 221 except on wider 512-bit data. The result may bewritten into an instruction specified register of global vector registerfile 231, L2/S2 local register file 222, M2/N2/C local register file 233or predicate register file 234.

Vector datapath side B 116 includes S2 unit 242. S2 unit 242 generallyaccepts two 512-bit operands and produces one 512-bit result. The twooperands are each recalled from an instruction specified register ineither global vector register file 231, L2/S2 local register file 232 orpredicate register file 234. S2 unit 242 preferably performsinstructions similar to S1 unit 222 except on wider 512-bit data. Theresult may be written into an instruction specified register of globalvector register file 231, L2/S2 local register file 222, M2/N2/C localregister file 233 or predicate register file 234. There may be certaindouble operations (called dual issued instructions) that employ both L2unit 241 and the S2 unit 242 together. The result may be written into aninstruction specified register of global vector register file 231, L2/S2local register file 232 or M2/N2/C local register file 233.

Vector datapath side B 116 includes M2 unit 243. M2 unit 243 generallyaccepts two 512-bit operands and produces one 512-bit result. The twooperands are each recalled from an instruction specified register ineither global vector register file 231 or M2/N2/C local register file233. M2 unit 243 preferably performs instructions similar to M1 unit 222except on wider 512-bit data. The result may be written into aninstruction specified register of global vector register file 231, L2/S2local register file 232 or M2/N2/C local register file 233.

Vector datapath side B 116 includes N2 unit 244. N2 unit 244 generallyaccepts two 512-bit operands and produces one 512-bit result. The twooperands are each recalled from an instruction specified register ineither global vector register file 231 or M2/N2/C local register file233. N2 unit 244 preferably performs the same type operations as M2 unit243. There may be certain double operations (called dual issuedinstructions) that employ both M2 unit 243 and the N2 unit 244 together.The result may be written into an instruction specified register ofglobal vector register file 231, L2/S2 local register file 232 orM2/N2/C local register file 233.

Vector datapath side B 116 includes C unit 245. C unit 245 generallyaccepts two 512-bit operands and produces one 512-bit result. The twooperands are each recalled from an instruction specified register ineither global vector register file 231 or M2/N2/C local register file233. C unit 245 preferably performs: “Rake” and “Search” instructions;up to 512 2-bit PN*8-bit multiplies I/Q complex multiplies per clockcycle; 8-bit and 16-bit Sum-of-Absolute-Difference (SAD) calculations,up to 512 SADs per clock cycle; horizontal add and horizontal min/maxinstructions; and vector permutes instructions. C unit 245 includes alsocontains 4 vector control registers (CUCR0 to CUCR3) used to controlcertain operations of C unit 245 instructions. Control registers CUCR0to CUCR3 are used as operands in certain C unit 245 operations. Controlregisters CUCR0 to CUCR3 are preferably used: in control of a generalpermutation instruction (VPERM); and as masks for SIMD multiple DOTproduct operations (DOTPM) and SIMD multiple Sum-of-Absolute-Difference(SAD) operations. Control register CUCR0 is preferably used to store thepolynomials for Galios Field Multiply operations (GFMPY). Controlregister CUCR1 is preferably used to store the Galois field polynomialgenerator function.

Vector datapath side B 116 includes P unit 246. P unit 246 performsbasic logic operations on registers of local predicate register file234. P unit 246 has direct access to read from and write to predicationregister file 234. These operations include AND, ANDN, OR, XOR, NOR,BITR, NEG, SET, BITCNT, RMBD, BIT Decimate and Expand. A commonlyexpected use of P unit 246 includes manipulation of the SIMD vectorcomparison results for use in control of a further SIMD vectoroperation.

FIG. 3 illustrates global scalar register file 211. There are 16independent 64-bit wide scalar registers designated A0 to A15. Eachregister of global scalar register file 211 can be read from or writtento as 64-bits of scalar data. All scalar datapath side A 115 functionalunits (L1 unit 221, S1 unit 222, M1 unit 223, N1 unit 224, D1 unit 225and D2 unit 237) can read or write to global scalar register file 211.Global scalar register file 211 may be read as 32-bits or as 64-bits andmay only be written to as 64-bits. The instruction executing determinesthe read data size. Vector datapath side B 116 functional units (L2 unit241, S2 unit 242, M2 unit 243, N2 unit 244, C unit 245 and P unit 246)can read from global scalar register file 211 via crosspath 117 underrestrictions that will be detailed below.

FIG. 4 illustrates D1/D2 local register file 214. There are 16independent 64-bit wide scalar registers designated D0 to D16. Eachregister of D1/D2 local register file 214 can be read from or written toas 64-bits of scalar data. All scalar datapath side A 115 functionalunits (L1 unit 221, unit 222, M1 unit 223, N1 unit 224, D1 unit 225 andD2 unit 237) can write to global scalar register file 211. Only D1 unit225 and D2 unit 237 can read from D1/D1 local scalar register file 214.It is expected that data stored in D1/D2 local scalar register file 214will include base addresses and offset addresses used in addresscalculation.

FIG. 5 illustrates L1/S1 local register file 212. The embodimentillustrated in FIG. 5 has 8 independent 64-bit wide scalar registersdesignated AL0 to AL7. The preferred instruction coding (see FIG. 13)permits L1/S1 local register file 212 to include up to 16 registers. Theembodiment of FIG. 5 implements only 8 registers to reduce circuit sizeand complexity. Each register of L1/S1 local register file 212 can beread from or written to as 64-bits of scalar data. All scalar datapathside A 115 functional units (L1 unit 221, S1 unit 222, M1 unit 223, N1unit 224, D1 unit 225 and D2 unit 237) can write to L1/S1 local scalarregister file 212. Only L1 unit 221 and S1 unit 222 can read from L1/S1local scalar register file 212.

FIG. 6 illustrates M1/N1 local register file 213. The embodimentillustrated in FIG. 6 has 8 independent 64-bit wide scalar registersdesignated AM0 to AM7. The preferred instruction coding (see FIG. 13)permits M1/N1 local register file 213 to include up to 16 registers. Theembodiment of FIG. 6 implements only 8 registers to reduce circuit sizeand complexity. Each register of M1/N1 local register file 213 can beread from or written to as 64-bits of scalar data. All scalar datapathside A 115 functional units (L1 unit 221, S1 unit 222, M1 unit 223, N1unit 224, D1 unit 225 and D2 unit 237) can write to M1/N1 local scalarregister file 213. Only M1 unit 223 and N1 unit 224 can read from M1/N1local scalar register file 213.

FIG. 7 illustrates global vector register file 231. There are 16independent 512-bit wide scalar registers. Each register of globalvector register file 231 can be read from or written to as 64-bits ofscalar data designated B0 to B15. Each register of global vectorregister file 231 can be read from or written to as 512-bits of vectordata designated VB0 to VB15. The instruction type determines the datasize. All vector datapath side B 116 functional units (L2 unit 241, S2unit 242, M3 unit 243, N2 unit 244, C unit 245 and P unit 246) can reador write to global scalar register file 231. Scalar datapath side A 115functional units (L1 unit 221, S1 unit 222, M1 unit 223, N1 unit 224, D1unit 225 and D2 unit 237) can read from global vector register file 231via crosspath 117 under restrictions that will be detailed below.

FIG. 8 illustrates P local register file 234. There are 8 independent64-bit wide registers designated P0 to P15. Each register of P localregister file 234 can be read from or written to as 64-bits of scalardata. Vector datapath side B 116 functional units L2 unit 241, S2 unit242, C unit 244 and P unit 246 can write to P local register file 234.Only L2 unit 241, S2 unit 242 and P unit 246 can read from P localscalar register file 234. A commonly expected use of P local registerfile 234 includes: writing one bit SIMD vector comparison results fromL2 unit 241, S2 unit 242 or C unit 244; manipulation of the SIMD vectorcomparison results by P unit 246; and use of the manipulated results incontrol of a further SIMD vector operation.

FIG. 9 illustrates L2/S2 local register file 232. The embodimentillustrated in FIG. 9 has 8 independent 512-bit wide scalar registers.The preferred instruction coding (see FIG. 13) permits L2/S2 localregister file 232 to include up to 16 registers. The embodiment of FIG.9 implements only 8 registers to reduce circuit size and complexity.Each register of L2/S2 local vector register file 232 can be read fromor written to as 64-bits of scalar data designated BL0 to BL7. Eachregister of L2/S2 local vector register file 232 can be read from orwritten to as 512-bits of vector data designated VBL0 to VBL7. Theinstruction type determines the data size. All vector datapath side B116 functional units (L2 unit 241, S2 unit 242, M2 unit 233, N2 unit244, C unit 245 and P unit 246) can write to L2/S2 local vector registerfile 232. Only L2 unit 241 and S2 unit 242 can read from L2/S2 localvector register file 232.

FIG. 10 illustrates M2/N2/C local register file 233. The embodimentillustrated in FIG. 10 has 8 independent 512-bit wide scalar registers.The preferred instruction coding (see FIG. 13) permits L1/S1 localregister file 212 to include up to 16 registers. The embodiment of FIG.10 implements only 8 registers to reduce circuit size and complexity.Each register of M2/N2/C local vector register file 233 can be read fromor written to as 64-bits of scalar data designated BM0 to BM7. Eachregister of M2/N2/C local vector register file 233 can be read from orwritten to as 512-bits of vector data designated VBM0 to VBM7. Allvector datapath side B 116 functional units (L2 unit 241, S2 unit 242,M2 unit 243, N2 unit 244, C unit 245 and P unit 246) can write toM2/N2/C local vector register file 233. Only M2 unit 233, N2 unit 244and C unit 245 can read from M2/N2/C local vector register file 233.

The provision of global register files accessible by all functionalunits of a side and local register files accessible by only some of thefunctional units of a side is a design choice. This invention could bepracticed employing only one type of register file corresponding to thedisclosed global register files.

Crosspath 117 permits limited exchange of data between scalar datapathside A 115 and vector datapath side B 116. During each operational cycleone 64-bit data word can be recalled from global scalar register file A211 for use as an operand by one or more functional units of vectordatapath side B 116 and one 64-bit data word can be recalled from globalvector register file 231 for use as an operand by one or more functionalunits of scalar datapath side A 115. Any scalar datapath side A 115functional unit (L1 unit 221, S1 unit 222, M1 unit 223, N1 unit 224, D1unit 225 and D2 unit 237) may read a 64-bit operand from global vectorregister file 231. This 64-bit operand is the least significant bits ofthe 512-bit data in the accessed register of global vector register file232. Plural scalar datapath side A 115 functional units may employ thesame 64-bit crosspath data as an operand during the same operationalcycle. However, only one 64-bit operand is transferred from vectordatapath side B 116 to scalar datapath side A 115 in any singleoperational cycle. Any vector datapath side B 116 functional unit (L2unit 241, S2 unit 242, M2 unit 243, N2 unit 244, C unit 245 and P unit246) may read a 64-bit operand from global scalar register file 211. Ifthe corresponding instruction is a scalar instruction, the crosspathoperand data is treated as any other 64-bit operand. If thecorresponding instruction is a vector instruction, the upper 448 bits ofthe operand are zero filled. Plural vector datapath side B 116functional units may employ the same 64-bit crosspath data as an operandduring the same operational cycle. Only one 64-bit operand istransferred from scalar datapath side A 115 to vector datapath side B116 in any single operational cycle.

Streaming engine 125 transfers data in certain restricted circumstances.Streaming engine 125 controls two data streams. A stream consists of asequence of elements of a particular type. Programs that operate onstreams read the data sequentially, operating on each element in turn.Every stream has the following basic properties. The stream data have awell-defined beginning and ending in time. The stream data have fixedelement size and type throughout the stream. The stream data have fixedsequence of elements. Thus programs cannot seek randomly within thestream. The stream data is read-only while active. Programs cannot writeto a stream while simultaneously reading from it. Once a stream isopened streaming engine 125: calculates the address; fetches the defineddata type from level two unified cache (which may require cache servicefrom a higher level memory); performs data type manipulation such aszero extension, sign extension, data element sorting/swapping such asmatrix transposition; and delivers the data directly to the programmeddata register file within central processing unit core 110. Streamingengine 125 is thus useful for real-time digital filtering operations onwell-behaved data. Streaming engine 125 frees these memory fetch tasksfrom the corresponding central processing unit core 110 enabling otherprocessing functions. Streaming engine 125 provides the followingbenefits.

Streaming engine 125 permits multi-dimensional memory accesses.Streaming engine 125 increases the available bandwidth to the functionalunits. Streaming engine 125 minimizes the number of cache miss stallssince the stream buffer bypasses level one data cache 123. Streamingengine 125 reduces the number of scalar operations required to maintaina loop. Streaming engine 125 manages address pointers. Streaming engine125 handles address generation automatically freeing up the addressgeneration instruction slots and Dl unit 224 and D2 unit 237 for othercomputations.

Central processing unit core 110 operates on an instruction pipeline.Instructions are fetched in instruction packets of fixed length furtherdescribed below. All instructions require the same number of pipelinephases for fetch and decode, but require a varying number of executephases.

FIG. 11 illustrates the following pipeline phases: program fetch phase1110, dispatch and decode phases 1120 and execution phases 1130. Programfetch phase 1110 includes three stages for all instructions. Dispatchand decode phases 1120 include three stages for all instructions.Execution phase 1130 includes one to four stages dependent on theinstruction.

Fetch phase 1110 includes program address generation stage 1111 (PG),program access stage 1112 (PA) and program receive stage 1113 (PR).During program address generation stage 1111 (PG), the program addressis generated in central processing unit core 110 and the read request issent to the memory controller for the level one instruction cache L1I.During the program access stage 1112 (PA) the level one instructioncache L1I processes the request, accesses the data in its memory andsends a fetch packet to the central processing unit core 110 boundary.During the program receive stage 1113 (PR) central processing unit core110 registers the fetch packet.

Instructions are always fetched sixteen 32-bit wide slots, constitutinga fetch packet, at a time. FIG. 12 illustrates 16 instructions 1201 to1216 of a single fetch packet. Fetch packets are aligned on 512-bit(16-word) boundaries. The preferred embodiment employs a fixed 32-bitinstruction length. Fixed length instructions are advantageous forseveral reasons. Fixed length instructions enable easy decoderalignment. A properly aligned instruction fetch can load pluralinstructions into parallel instruction decoders. Such a properly alignedinstruction fetch can be achieved by predetermined instruction alignmentwhen stored in memory (fetch packets aligned on 512-bit boundaries)coupled with a fixed instruction packet fetch. An aligned instructionfetch permits operation of parallel decoders on instruction-sizedfetched bits. Variable length instructions require an initial step oflocating each instruction boundary before they can be decoded. A fixedlength instruction set generally permits more regular layout ofinstruction fields. This simplifies the construction of each decoderwhich is an advantage for a wide issue VLIW central processor.

The execution of the individual instructions is partially controlled bya p bit in each instruction. This p bit is preferably bit 0 of the32-bit wide slot. The p bit determines whether an instruction executesin parallel with a next instruction. Instructions are scanned from lowerto higher address. If the p bit of an instruction is 1, then the nextfollowing instruction (higher memory address) is executed in parallelwith (in the same cycle as) that instruction. If the p bit of aninstruction is 0, then the next following instruction is executed in thecycle after the instruction.

Central processing unit core 110 and level one instruction cache L1I 121pipelines are de-coupled from each other. Fetch packet returns fromlevel one instruction cache L1I can take different number of clockcycles, depending on external circumstances such as whether there is ahit in level one instruction cache 121 or a hit in level two combinedcache 130. Therefore program access stage 1112 (PA) can take severalclock cycles instead of 1 clock cycle as in the other stages.

The instructions executing in parallel constitute an execute packet. Inthe preferred embodiment an execute packet can contain up to sixteeninstructions. No two instructions in an execute packet may use the samefunctional unit. A slot is one of five types: 1) a self-containedinstruction executed on one of the functional units of centralprocessing unit core 110 (L1 unit 221, S1 unit 222, M1 unit 223, N1 unit224, D1 unit 225, D2 unit 237, L2 unit 241, S2 unit 242, M2 unit 243, N2unit 244, C unit 245 and P unit 246); 2) a unitless instruction such asa NOP (no operation) instruction or multiple NOP instruction; 3) abranch instruction; 4) a constant field extension; and 5) a conditionalcode extension. Some of these slot types will be further explainedbelow.

Dispatch and decode phases 1110 include instruction dispatch toappropriate execution unit stage 1121 (DS), instruction pre-decode stage1122 (DC1); and instruction decode, operand reads stage 1123 (DC2).During instruction dispatch to appropriate execution unit stage 1121(DS) the fetch packets are split into execute packets and assigned tothe appropriate functional units. During the instruction pre-decodestage 1122 (DC1), the source registers, destination registers andassociated paths are decoded for the execution of the instructions inthe functional units. During the instruction decode, operand reads stage1123 (DC2), more detailed unit decodes are done, as well as readingoperands from the register files.

Execution phases 1130 includes execution stages 1131 to 1135 (E1 to E5).Different types of instructions require different numbers of thesestages to complete their execution. These stages of the pipeline play animportant role in understanding the device state at central processingunit core 110 cycle boundaries.

During execute 1 stage 1131 (E1) the conditions for the instructions areevaluated and operands are operated on. As illustrated in FIG. 11,execute 1 stage 1131 may receive operands from a stream buffer 1141 andone of the register files shown schematically as 1142. For load andstore instructions, address generation is performed and addressmodifications are written to a register file. For branch instructions,branch fetch packet in PG phase 1111 is affected. As illustrated in FIG.11, load and store instructions access memory here shown schematicallyas memory 1151. For single-cycle instructions, results are written to adestination register file. This assumes that any conditions for theinstructions are evaluated as true. If a condition is evaluated asfalse, the instruction does not write any results or have any pipelineoperation after execute 1 stage 1131.

During execute 2 stage 1132 (E2) load instructions send the address tomemory. Store instructions send the address and data to memory.Single-cycle instructions that saturate results set the SAT bit in thecontrol status register (CSR) if saturation occurs. For 2-cycleinstructions, results are written to a destination register file.

During execute 3 stage 1133 (E3) data memory accesses are performed. Anymultiply instructions that saturate results set the SAT bit in thecontrol status register (CSR) if saturation occurs. For 3-cycleinstructions, results are written to a destination register file.

During execute 4 stage 1134 (E4) load instructions bring data to thecentral processing unit core 110 boundary. For 4-cycle instructions,results are written to a destination register file.

During execute 5 stage 1135 (E5) load instructions write data into aregister. This is illustrated schematically in FIG. 11 with input frommemory 1151 to execute 5 stage 1135.

FIG. 13 illustrates an example of the instruction coding 1300 offunctional unit instructions used by this invention. Those skilled inthe art would realize that other instruction codings are feasible andwithin the scope of this invention. Each instruction consists of 32 bitsand controls the operation of one of the individually controllablefunctional units (L1 unit 221, S1 unit 222, M1 unit 223, N1 unit 224, D1unit 225, D2 unit 237, L2 unit 241, S2 unit 242, M2 unit 243, N2 unit244, C unit 245 and P unit 246). The bit fields are defined as follows.

The creg field 1301 (bits 29 to 31) and the z bit 1302 (bit 28) areoptional fields used in conditional instructions. These bits are usedfor conditional instructions to identify the predicate register and thecondition. The z bit 1302 (bit 28) indicates whether the predication isbased upon zero or not zero in the predicate register. If z=1, the testis for equality with zero. If z=0, the test is for nonzero. The case ofcreg=0 and z=0 is treated as always true to allow unconditionalinstruction execution. The creg field 1301 and the z field 1302 areencoded in the instruction as shown in Table 1.

TABLE 1 Conditional creg z Register 31 30 29 28 Unconditional 0 0 0 0Reserved 0 0 0 1 A0 0 0 1 z A1 0 1 0 z A2 0 1 1 z A3 1 0 0 z A4 1 0 1 zA5 1 1 0 z Reserved 1 1 x xExecution of a conditional instruction is conditional upon the valuestored in the specified data register. This data register is in theglobal scalar register file 211 for all functional units. Note that “z”in the z bit column refers to the zero/not zero comparison selectionnoted above and “x” is a don't care state. This coding can only specifya subset of the 16 global registers as predicate registers. Thisselection was made to preserve bits in the instruction coding. Note thatunconditional instructions do not have these optional bits. Forunconditional instructions these bits in fields 1301 and 1302 (28 to 31)are preferably used as additional opcode bits.

The dst field 1303 (bits 23 to 27) specifies a register in acorresponding register file as the destination of the instructionresults.

The src2/cst field 1304 (bits 18 to 22) has several meanings dependingon the instruction opcode field (bits 3 to 12 for all instructions andadditionally bits 28 to 31 for unconditional instructions). The firstmeaning specifies a register of a corresponding register file as thesecond operand. The second meaning is an immediate constant. Dependingon the instruction type, this is treated as an unsigned integer and zeroextended to a specified data length or is treated as a signed integerand sign extended to the specified data length.

The src1 field 1305 (bits 13 to 17) specifies a register in acorresponding register file as the first source operand.

The opcode field 1306 (bits 3 to 12) for all instructions (andadditionally bits 28 to 31 for unconditional instructions) specifies thetype of instruction and designates appropriate instruction options. Thisincludes unambiguous designation of the functional unit used andoperation performed. A detailed explanation of the opcode is beyond thescope of this invention except for the instruction options detailedbelow.

The e bit 1307 (bit 2) is only used for immediate constant instructionswhere the constant may be extended. If e=1, then the immediate constantis extended in a manner detailed below. If e=0, then the immediateconstant is not extended. In that case the immediate constant isspecified by the src2/cst field 1304 (bits 18 to 22). Note that this ebit 1307 is used for only some instructions. Accordingly, with propercoding this e bit 1307 may be omitted from instructions which do notneed it and this bit used as an additional opcode bit.

The s bit 1307 (bit 1) designates scalar datapath side A 115 or vectordatapath side B 116. If s=0, then scalar datapath side A 115 isselected. This limits the functional unit to L1 unit 221, S1 unit 222,M1 unit 223, N1 unit 224, D1 unit 225 and D2 unit 237 and thecorresponding register files illustrated in FIG. 2. Similarly, s=1selects vector datapath side B 116 limiting the functional unit to L2unit 241, S2 unit 242, M2 unit 243, N2 unit 244, P unit 246 and thecorresponding register file illustrated in FIG. 2.

The p bit 1308 (bit 0) marks the execute packets. The p-bit determineswhether the instruction executes in parallel with the followinginstruction. The p-bits are scanned from lower to higher address. If p=1for the current instruction, then the next instruction executes inparallel with the current instruction. If p=0 for the currentinstruction, then the next instruction executes in the cycle after thecurrent instruction. All instructions executing in parallel constitutean execute packet. An execute packet can contain up to twelveinstructions. Each instruction in an execute packet must use a differentfunctional unit.

There are two different condition code extension slots. Each executepacket can contain one each of these unique 32-bit condition codeextension slots which contains the 4-bit creg/z fields for theinstructions in the same execute packet. FIG. 14 illustrates the codingfor condition code extension slot 0 and FIG. 15 illustrates the codingfor condition code extension slot 1.

FIG. 14 illustrates the coding for condition code extension slot 0having 32 bits. Field 1401 (bits 28 to 31) specify 4 creg/z bitsassigned to the L1 unit 221 instruction in the same execute packet.Field 1402 (bits 27 to 24) specify 4 creg/z bits assigned to the L2 unit241 instruction in the same execute packet. Field 1403 (bits 19 to 23)specify 4 creg/z bits assigned to the S1 unit 222 instruction in thesame execute packet. Field 1404 (bits 16 to 19) specify 4 creg/z bitsassigned to the S2 unit 242 instruction in the same execute packet.Field 1405 (bits 12 to 15) specify 4 creg/z bits assigned to the D1 unit225 instruction in the same execute packet. Field 1406 (bits 8 to 11)specify 4 creg/z bits assigned to the D2 unit 245 instruction in thesame execute packet. Field 1407 (bits 6 and 7) is unused/reserved. Field1408 (bits 0 to 5) are coded a set of unique bits (CCEX0) to identifythe condition code extension slot 0. Once this unique ID of conditioncode extension slot 0 is detected, the corresponding creg/z bits areemployed to control conditional execution of any L1 unit 221, L2 unit241, S1 unit 222, S2 unit 242, D1 unit 224 and D2 unit 225 instructionin the same execution packet. These creg/z bits are interpreted as shownin Table 1. If the corresponding instruction is conditional (includescreg/z bits) the corresponding bits in the condition code extension slot0 override the condition code bits in the instruction. Note that noexecution packet can have more than one instruction directed to aparticular execution unit. No execute packet of instructions can containmore than one condition code extension slot 0. Thus the mapping ofcreg/z bits to functional unit instruction is unambiguous. Setting thecreg/z bits equal to “0000” makes the instruction unconditional. Thus aproperly coded condition code extension slot 0 can make somecorresponding instructions conditional and some unconditional.

FIG. 15 illustrates the coding for condition code extension slot 1having 32 bits. Field 1501 (bits 28 to 31) specify 4 creg/z bitsassigned to the M1 unit 223 instruction in the same execute packet.Field 1502 (bits 27 to 24) specify 4 creg/z bits assigned to the M2 unit243 instruction in the same execute packet. Field 1503 (bits 19 to 23)specify 4 creg/z bits assigned to the C unit 245 instruction in the sameexecute packet. Field 1504 (bits 16 to 19) specify 4 creg/z bitsassigned to the N1 unit 224 instruction in the same execute packet.Field 1505 (bits 12 to 15) specify 4 creg/z bits assigned to the N2 unit244 instruction in the same execute packet. Field 1506 (bits 5 to 11) isunused/reserved. Field 1507 (bits 0 to 5) are coded a set of unique bits(CCEX1) to identify the condition code extension slot 1. Once thisunique ID of condition code extension slot 1 is detected, thecorresponding creg/z bits are employed to control conditional executionof any M1 unit 223, M2 unit 243, C unit 245, N1 unit 224 and N2 unit 244instruction in the same execution packet. These creg/z bits areinterpreted as shown in Table 1. If the corresponding instruction isconditional (includes creg/z bits) the corresponding bits in thecondition code extension slot 1 override the condition code bits in theinstruction. Note that no execution packet can have more than oneinstruction directed to a particular execution unit. No execute packetof instructions can contain more than one condition code extension slot1. Thus the mapping of creg/z bits to functional unit instruction isunambiguous. Setting the creg/z bits equal to “0000” makes theinstruction unconditional. Thus a properly coded condition codeextension slot 1 can make some instructions conditional and someunconditional.

It is feasible for both condition code extension slot 0 and conditioncode extension slot 1 to include a p bit to define an execute packet asdescribed above in conjunction with FIG. 13. In the preferredembodiment, as illustrated in FIGS. 14 and 15, code extension slot 0 andcondition code extension slot 1 preferably have bit 0 (p bit) alwaysencoded as 1. Thus neither condition code extension slot 0 nor conditioncode extension slot 1 can be in the last instruction slot of an executepacket.

There are two different constant extension slots. Each execute packetcan contain one each of these unique 32-bit constant extension slotswhich contains 27 bits to be concatenated as high order bits with the5-bit constant field 1305 to form a 32-bit constant. As noted in theinstruction coding description above only some instructions define thesrc2/cst field 1304 as a constant rather than a source registeridentifier. At least some of those instructions may employ a constantextension slot to extend this constant to 32 bits.

FIG. 16 illustrates the fields of constant extension slot 0. Eachexecute packet may include one instance of constant extension slot 0 andone instance of constant extension slot 1. FIG. 16 illustrates thatconstant extension slot 0 1600 includes two fields. Field 1601 (bits 5to 31) constitute the most significant 27 bits of an extended 32-bitconstant including the target instruction scr2/cst field 1304 as thefive least significant bits. Field 1602 (bits 0 to 4) are coded a set ofunique bits (CSTX0) to identify the constant extension slot 0. In thepreferred embodiment constant extension slot 0 1600 can only be used toextend the constant of one of an L1 unit 221 instruction, data in a D1unit 225 instruction, an S2 unit 242 instruction, an offset in a D2 unit237 instruction, an M2 unit 243 instruction, an N2 unit 244 instruction,a branch instruction, or a C unit 245 instruction in the same executepacket. Constant extension slot 1 is similar to constant extension slot0 except that bits 0 to 4 are coded a set of unique bits (CSTX1) toidentify the constant extension slot 1. In the preferred embodimentconstant extension slot 1 can only be used to extend the constant of oneof an L2 unit 241 instruction, data in a D2 unit 237 instruction, an S1unit 222 instruction, an offset in a D1 unit 225 instruction, an M1 unit223 instruction or an N1 unit 224 instruction in the same executepacket.

Constant extension slot 0 and constant extension slot 1 are used asfollows. The target instruction must be of the type permitting constantspecification. As known in the art this is implemented by replacing oneinput operand register specification field with the least significantbits of the constant as described above with respect to scr2/cst field1304. Instruction decoder 113 determines this case, known as animmediate field, from the instruction opcode bits. The targetinstruction also includes one constant extension bit (e bit 1307)dedicated to signaling whether the specified constant is not extended(preferably constant extension bit=0) or the constant is extended(preferably constant extension bit=1). If instruction decoder 113detects a constant extension slot 0 or a constant extension slot 1, itfurther checks the other instructions within that execute packet for aninstruction corresponding to the detected constant extension slot. Aconstant extension is made only if one corresponding instruction has aconstant extension bit (e bit 1307) equal to 1.

FIG. 17 is a partial block diagram 1700 illustrating constant extension.FIG. 17 assumes that instruction decoder 113 detects a constantextension slot and a corresponding instruction in the same executepacket. Instruction decoder 113 supplies the 27 extension bits from theconstant extension slot (bit field 1601) and the 5 constant bits (bitfield 1305) from the corresponding instruction to concatenator 1701.Concatenator 1701 forms a single 32-bit word from these two parts. Inthe preferred embodiment the 27 extension bits from the constantextension slot (bit field 1601) are the most significant bits and the 5constant bits (bit field 1305) are the least significant bits. Thiscombined 32-bit word is supplied to one input of multiplexer 1702. The 5constant bits from the corresponding instruction field 1305 supply asecond input to multiplexer 1702. Selection of multiplexer 1702 iscontrolled by the status of the constant extension bit. If the constantextension bit (e bit 1307) is 1 (extended), multiplexer 1702 selects theconcatenated 32-bit input. If the constant extension bit is 0 (notextended), multiplexer 1702 selects the 5 constant bits from thecorresponding instruction field 1305. Multiplexer 1702 supplies thisoutput to an input of sign extension unit 1703.

Sign extension unit 1703 forms the final operand value from the inputfrom multiplexer 1703. Sign extension unit 1703 receives control inputsScalar/Vector and Data Size. The Scalar/Vector input indicates whetherthe corresponding instruction is a scalar instruction or a vectorinstruction. The functional units of data path side A 115 (L1 unit 221,S1 unit 222, M1 unit 223, N1 unit 224, D1 unit 225 and D2 unit 237) canonly perform scalar instructions. Any instruction directed to one ofthese functional units is a scalar instruction. Data path side Bfunctional units L2 unit 241, S2 unit 242, M2 unit 243, N2 unit 244 andC unit 245 may perform scalar instructions or vector instructions.Instruction decoder 113 determines whether the instruction is a scalarinstruction or a vector instruction from the opcode bits. P unit 246 mayonly preform scalar instructions. The Data Size may be 8 bits (byte B),16 bits (half-word H), 32 bits (word W) or 64 bits (double word D).

Table 2 lists the operation of sign extension unit 1703 for the variousoptions.

TABLE 2 Instruction Operand Constant Type Size Length Action ScalarB/H/W/D 5 bits Sign extend to 64 bits Scalar B/H/W/D 32 bits Sign extendto 64 bits Vector B/H/W/D 5 bits Sign extend to operand size andreplicate across whole vector Vector B/H/W 32 bits Replicate 32-bitconstant across each 32-bit (W) lane Vector D 32 bits Sign extend to 64bits and replicate across each 64-bit (D) lane

It is feasible for both constant extension slot 0 and constant extensionslot 1 to include a p bit to define an execute packet as described abovein conjunction with FIG. 13. In the preferred embodiment, as in the caseof the condition code extension slots, constant extension slot 0 andconstant extension slot 1 preferably have bit 0 (p bit) always encodedas 1. Thus neither constant extension slot 0 nor constant extension slot1 can be in the last instruction slot of an execute packet.

It is technically feasible for an execute packet to include a constantextension slot 0 or 1 and more than one corresponding instruction markedconstant extended (e bit=1). For constant extension slot 0 this wouldmean more than one of an L1 unit 221 instruction, data in a D1 unit 225instruction, an S2 unit 242 instruction, an offset in a D2 unit 237instruction, an M2 unit 243 instruction or an N2 unit 244 instruction inan execute packet have an e bit of 1. For constant extension slot 1 thiswould mean more than one of an L2 unit 241 instruction, data in a D2unit 237 instruction, an S1 unit 222 instruction, an offset in a D1 unit225 instruction, an M1 unit 223 instruction or an N1 unit 224instruction in an execute packet have an e bit of 1. Supplying the sameconstant extension to more than one instruction is not expected to be auseful function. Accordingly, in one embodiment instruction decoder 113may determine this case an invalid operation and not supported.Alternately, this combination may be supported with extension bits ofthe constant extension slot applied to each corresponding functionalunit instruction marked constant extended.

Special vector predicate instructions use registers in predicateregister file 234 to control vector operations. In the currentembodiment all these SIMD vector predicate instructions operate onselected data sizes. The data sizes may include byte (8 bit) data, halfword (16 bit) data, word (32 bit) data, double word (64 bit) data, quadword (128 bit) data and half vector (256 bit) data. Each bit of thepredicate register controls whether a SIMD operation is performed uponthe corresponding byte of data. The operations of P unit 245 permit avariety of compound vector SIMD operations based upon more than onevector comparison. For example a range determination can be made usingtwo comparisons. A candidate vector is compared with a first vectorreference having the minimum of the range packed within a first dataregister. A second comparison of the candidate vector is made with asecond reference vector having the maximum of the range packed within asecond data register. Logical combinations of the two resultingpredicate registers would permit a vector conditional operation todetermine whether each data part of the candidate vector is within rangeor out of range.

L1 unit 221, S1 unit 222, L2 unit 241, S2 unit 242 and C unit 245 oftenoperate in a single instruction multiple data (SIMD) mode. In this SIMDmode the same instruction is applied to packed data from the twooperands. Each operand holds plural data elements disposed inpredetermined slots. SIMD operation is enabled by carry control at thedata boundaries. Such carry control enables operations on varying datawidths.

FIG. 18 illustrates the carry control. AND gate 1801 receives the carryoutput of bit N within the operand wide arithmetic logic unit (64 bitsfor scalar datapath side A 115 functional units and 512 bits for vectordatapath side B 116 functional units). AND gate 1801 also receives acarry control signal which will be further explained below. The outputof AND gate 1801 is supplied to the carry input of bit N+1 of theoperand wide arithmetic logic unit. AND gates such as AND gate 1801 aredisposed between every pair of bits at a possible data boundary. Forexample, for 8-bit data such an AND gate will be between bits 7 and 8,bits 15 and 16, bits 23 and 24, etc. Each such AND gate receives acorresponding carry control signal. If the data size is of the minimum,then each carry control signal is 0, effectively blocking carrytransmission between the adjacent bits. The corresponding carry controlsignal is 1 if the selected data size requires both arithmetic logicunit sections. Table 3 below shows example carry control signals for thecase of a 512 bit wide operand such as used by vector datapath side B116 functional units which may be divided into sections of 8 bits, 16bits, 32 bits, 64 bits, 128 bits or 256 bits. In Table 3 the upper 32bits control the upper bits (bits 128 to 511) carries and the lower 32bits control the lower bits (bits 0 to 127) carries. No control of thecarry output of the most significant bit is needed, thus only 63 carrycontrol signals are required.

TABLE 3 Data Size Carry Control Signals 8 bits (B) −000 0000 0000 00000000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 16 bits (H)−101 0101 0101 0101 0101 0101 0101 0101 0101 0101 0101 0101 0101 01010101 0101 32 bits (W) −111 0111 0111 0111 0111 0111 0111 0111 0111 01110111 0111 0111 0111 0111 0111 64 bits (D) −111 1111 0111 1111 0111 11110111 1111 0111 1111 0111 1111 0111 1111 0111 1111 128 bits −111 11111111 1111 0111 1111 1111 1111 0111 1111 1111 1111 0111 1111 1111 1111256 bits −111 1111 1111 1111 1111 1111 1111 1111 0111 1111 1111 11111111 1111 1111 1111It is typical in the art to operate on data sizes that are integralpowers of 2 (2^(N)). However, this carry control technique is notlimited to integral powers of 2. One skilled in the art would understandhow to apply this technique to other data sizes and other operandwidths.

FIG. 19 illustrates a conceptual view of the streaming engines of thisinvention. FIG. 19 illustrates the process of a single stream. Streamingengine 1900 includes stream address generator 1901. Stream addressgenerator 1901 sequentially generates addresses of the elements of thestream and supplies these element addresses to system memory 1910.Memory 1910 recalls data stored at the element addresses (data elements)and supplies these data elements to data first-in-first-out (FIFO)memory 1902. Data FIFO 1902 provides buffering between memory 1910 andCPU 1920. Data formatter 1903 receives the data elements from data FIFOmemory 1902 and provides data formatting according to the streamdefinition. This process will be described below. Streaming engine 1900supplies the formatted data elements from data formatter 1903 to the CPU1920. The program on CPU 1920 consumes the data and generates an output.

Stream elements typically reside in normal memory. The memory itselfimposes no particular structure upon the stream. Programs define streamsand therefore impose structure, by specifying the following streamattributes: address of the first element of the stream; size and type ofthe elements in the stream; formatting for data in the stream; and theaddress sequence associated with the stream.

The streaming engine defines an address sequence for elements of thestream in terms of a pointer walking through memory. A multiple-levelnested loop controls the path the pointer takes. An iteration count fora loop level indicates the number of times that level repeats. Adimension gives the distance between pointer positions of that looplevel.

In a basic forward stream the innermost loop always consumes physicallycontiguous elements from memory. The implicit dimension of thisinnermost loop is 1 element. The pointer itself moves from element toelement in consecutive, increasing order. In each level outside theinner loop, that loop moves the pointer to a new location based on thesize of that loop level's dimension.

This form of addressing allows programs to specify regular paths throughmemory in a small number of parameters. Table 4 lists the addressingparameters of a basic stream.

TABLE 4 Parameter Definition ELEM_BYTES Size of each element in bytesICNT0 Number of iterations for the innermost loop level 0. At loop level0 all elements are physically contiguous DIM0 is ELEM_BYTES ICNT1 Numberof iterations for loop level 1 DIM1 Number of bytes between the startingpoints for consecutive iterations of loop level 1 ICNT2 Number ofiterations for loop level 2 DIM2 Number of bytes between the startingpoints for consecutive iterations of loop level 2 ICNT3 Number ofiterations for loop level 3 DIM3 Number of bytes between the startingpoints for consecutive iterations of loop level 3

The definition above maps consecutive elements of the stream toincreasing addresses in memory. This works well for most algorithms butnot all. Some algorithms are better served by reading elements indecreasing memory addresses, reverse stream addressing. For example, adiscrete convolution computes vector dot-products, as per the formula:

${\left( {f,g} \right)\lbrack t\rbrack} = {\sum\limits_{x = {- \infty}}^{\infty}{{f\lbrack x\rbrack}{g\left\lbrack {t - x} \right\rbrack}}}$In most DSP code, f[ ] and g[ ] represent arrays in memory. For eachoutput, the algorithm reads f[ ] in the forward direction, but reads g[] in the reverse direction. Practical filters limit the range of indicesfor [x] and [t-x] to a finite number elements. To support this pattern,the streaming engine supports reading elements in decreasing addressorder.

Matrix multiplication presents a unique problem to the streaming engine.Each element in the matrix product is a vector dot product between a rowfrom the first matrix and a column from the second. Programs typicallystore matrices all in row-major or column-major order. Row-major orderstores all the elements of a single row contiguously in memory.Column-major order stores all elements of a single column contiguouslyin memory. Matrices typically get stored in the same order as thedefault array order for the language. As a result, only one of the twomatrices in a matrix multiplication map on to the streaming engine's2-dimensional stream definition. In a typical example a first indexsteps through columns on array first array but rows on second array.This problem is not unique to the streaming engine. Matrixmultiplication's access pattern fits poorly with most general-purposememory hierarchies. Some software libraries transposed one of the twomatrices, so that both get accessed row-wise (or column-wise) duringmultiplication. The streaming engine supports implicit matrixtransposition with transposed streams. Transposed streams avoid the costof explicitly transforming the data in memory. Instead of accessing datain strictly consecutive-element order, the streaming engine effectivelyinterchanges the inner two loop dimensions in its traversal order,fetching elements along the second dimension into contiguous vectorlanes.

This algorithm works, but is impractical to implement for small elementsizes. Some algorithms work on matrix tiles which are multiple columnsand rows together. Therefore, the streaming engine defines a separatetransposition granularity. The hardware imposes a minimum granularity.The transpose granularity must also be at least as large as the elementsize. Transposition granularity causes the streaming engine to fetch oneor more consecutive elements from dimension 0 before moving alongdimension 1. When the granularity equals the element size, this resultsin fetching a single column from a row-major array. Otherwise, thegranularity specifies fetching 2, 4 or more columns at a time from arow-major array. This is also applicable for column-major layout byexchanging row and column in the description. A parameter GRANULEindicates the transposition granularity in bytes.

Another common matrix multiplication technique exchanges the innermosttwo loops of the matrix multiply. The resulting inner loop no longerreads down the column of one matrix while reading across the row ofanother. For example the algorithm may hoist one term outside the innerloop, replacing it with the scalar value. On a vector machine, theinnermost loop can be implements very efficiently with a singlescalar-by-vector multiply followed by a vector add. The centralprocessing unit core 110 of this invention lacks a scalar-by-vectormultiply. Programs must instead duplicate the scalar value across thelength of the vector and use a vector-by-vector multiply. The streamingengine of this invention directly supports this and related use modelswith an element duplication mode. In this mode, the streaming enginereads a granule smaller than the full vector size and replicates thatgranule to fill the next vector output.

The streaming engine treats each complex number as a single element withtwo sub-elements that give the real and imaginary (rectangular) ormagnitude and angle (polar) portions of the complex number. Not allprograms or peripherals agree what order these sub-elements shouldappear in memory. Therefore, the streaming engine offers the ability toswap the two sub-elements of a complex number with no cost. This featureswaps the halves of an element without interpreting the contents of theelement and can be used to swap pairs of sub-elements of any type, notjust complex numbers.

Algorithms generally prefer to work at high precision, but highprecision values require more storage and bandwidth than lower precisionvalues. Commonly, programs will store data in memory at low precision,promote those values to a higher precision for calculation and thendemote the values to lower precision for storage. The streaming enginesupports this directly by allowing algorithms to specify one level oftype promotion. In the preferred embodiment of this invention everysub-element may be promoted to the next larger type size with eithersign or zero extension for integer types. It is also feasible that thestreaming engine may support floating point promotion, promoting 16-bitand 32-bit floating point values to 32-bit and 64-bit formats,respectively.

The streaming engine defines a stream as a discrete sequence of dataelements, the central processing unit core 110 consumes data elementspacked contiguously in vectors. Vectors resemble streams in as much asthey contain multiple homogeneous elements with some implicit sequence.Because the streaming engine reads streams, but the central processingunit core 110 consumes vectors, the streaming engine must map streamsonto vectors in a consistent way.

Vectors consist of equal-sized lanes, each lane containing asub-element. The central processing unit core 110 designates therightmost lane of the vector as lane 0, regardless of device's currentendian mode. Lane numbers increase right-to-left. The actual number oflanes within a vector varies depending on the length of the vector andthe data size of the sub-element.

FIG. 20 illustrates a first example of lane allocation in a vector.Vector 2000 is divided into 8 64-bit lanes (8×64 bits=512 bits thevector length). Lane 0 includes bits 0 to 63; line 1 includes bits 64 to125; lane 2 includes bits 128 to 191; lane 3 includes bits 192 to 255,lane 4 includes bits 256 to 319, lane 5 includes bits 320 to 383, lane 6includes bits 384 to 447 and lane 7 includes bits 448 to 511.

FIG. 21 illustrates a second example of lane allocation in a vector.Vector 2100 is divided into 16 32-bit lanes (16×32 bits=512 bits thevector length). Lane 0 includes bits 0 to 31; line 1 includes bits 32 to63; lane 2 includes bits 64 to 95; lane 3 includes bits 96 to 127; lane4 includes bits 128 to 159; lane 5 includes bits 160 to 191; lane 6includes bits 192 to 223; lane 7 includes bits 224 to 255; lane 8includes bits 256 to 287; line 9 occupied bits 288 to 319; lane 10includes bits 320 to 351; lane 11 includes bits 352 to 387; lane 12includes bits 388 to 415; lane 13 includes bits 416 to 447; lane 14includes bits 448 to 479; and lane 15 includes bits 480 to 511.

The streaming engine maps the innermost stream dimension directly tovector lanes. It maps earlier elements within that dimension to lowerlane numbers and later elements to higher lane numbers. This is trueregardless of whether this particular stream advances in increasing ordecreasing address order. Whatever order the stream defines, thestreaming engine deposits elements in vectors in increasing-lane order.For non-complex data, it places the first element in lane 0 of the firstvector central processing unit core 110 fetches, the second in lane 1,and so on. For complex data, the streaming engine places the firstelement in lanes 0 and 1, second in lanes 2 and 3, and so on.Sub-elements within an element retain the same relative orderingregardless of the stream direction. For non-swapped complex elements,this places the sub-elements with the lower address of each pair in theeven numbered lanes, and the sub-elements with the higher address ofeach pair in the odd numbered lanes. Swapped complex elements reversethis mapping.

The streaming engine fills each vector central processing unit core 110fetches with as many elements as it can from the innermost streamdimension. If the innermost dimension is not a multiple of the vectorlength, the streaming engine pads that dimension out to a multiple ofthe vector length with zeros. Thus for higher-dimension streams, thefirst element from each iteration of an outer dimension arrives in lane0 of a vector. The streaming engine always maps the innermost dimensionto consecutive lanes in a vector. For transposed streams, the innermostdimension consists of groups of sub-elements along dimension 1, notdimension 0, as transposition exchanges these two dimensions.

Two dimensional streams exhibit great variety as compared to onedimensional streams. A basic two dimensional stream extracts a smallerrectangle from a larger rectangle. A transposed 2-D stream reads arectangle column-wise instead of row-wise. A looping stream, where thesecond dimension overlaps first executes a finite impulse response (FIR)filter taps which loops repeatedly or FIR filter samples which provide asliding window of input samples.

FIG. 22 illustrates a basic two dimensional stream. The inner twodimensions, represented by ELEM_BYTES, ICNT0, DIM1 and ICNT1 givesufficient flexibility to describe extracting a smaller rectangle 2220having dimensions 2221 and 2222 from a larger rectangle 2210 havingdimensions 2211 and 2212. In this example rectangle 2220 is a 9 by 13rectangle of 64-bit values and rectangle 2210 is a larger 11 by 19rectangle. The following stream parameters define this stream:

-   -   ICNT0=9    -   ELEM_BYTES=8    -   ICNT1=13    -   DIM1=88 (11 times 8)        Thus the iteration count in the 0 dimension 2221 is 9. The        iteration count in the 1 direction 2222 is 13. Note that the        ELEM_BYTES only scales the innermost dimension. The first        dimension has ICNT0 elements of size ELEM_BYTES. The stream        address generator does not scale the outer dimensions.        Therefore, DIM1=88, which is 11 elements scaled by 8 bytes per        element.

FIG. 23 illustrates the order of elements within this example stream.The streaming engine fetches elements for the stream in the orderillustrated in order 2300. The first 9 elements come from the first rowof rectangle 2220, left-to-right in hops 1 to 8. The 10th through 24thelements comes from the second row, and so on. When the stream movesfrom the 9th element to the 10th element (hop 9 in FIG. 23), thestreaming engine computes the new location based on the pointer'sposition at the start of the inner loop, not where the pointer ended upat the end of the first dimension. This makes DIM1 independent ofELEM_BYTES and ICNT0. DIM1 always represents the distance between thefirst bytes of each consecutive row.

Transposed streams access along dimension 1 before dimension 0. Thefollowing examples illustrate a couple transposed streams, varying thetransposition granularity. FIG. 24 illustrates extracting a smallerrectangle 2420 (12×8) having dimensions 2421 and 2422 from a largerrectangle 2410 (14×13) having dimensions 2411 and 2412. In FIG. 24ELEM_BYTES equals 2.

FIG. 25 illustrates how the streaming engine would fetch the stream ofthis example with a transposition granularity of 4 bytes. Fetch pattern2500 fetches pairs of elements from each row (because the granularity of4 is twice the ELEM_BYTES of 2), but otherwise moves down the columns.Once it reaches the bottom of a pair of columns, it repeats this patternwith the next pair of columns.

FIG. 26 illustrates how the streaming engine would fetch the stream ofthis example with a transposition granularity of 8 bytes. The overallstructure remains the same. The streaming engine fetches 4 elements fromeach row (because the granularity of 8 is four times the ELEM_BYTES of2) before moving to the next row in the column as shown in fetch pattern2600.

The streams examined so far read each element from memory exactly once.A stream can read a given element from memory multiple times, in effectlooping over a piece of memory. FIR filters exhibit two common loopingpatterns. FIRs re-read the same filter taps for each output. FIRs alsoread input samples from a sliding window. Two consecutive outputs willneed inputs from two overlapping windows.

FIG. 27 illustrates the details of streaming engine 2700. Streamingengine 2700 contains three major sections: Stream 0 2710; Stream 1 2720;and Shared L2 Interfaces 2730. Stream 0 2710 and Stream 1 2720 bothcontain identical hardware that operates in parallel. Stream 0 2710 andStream 1 2720 both share L2 interfaces 2730. Each stream 2710 and 2720provides central processing unit core 110 with up to 512 bits/cycle,every cycle. The streaming engine architecture enables this through itsdedicated stream paths and shared dual L2 interfaces.

Each streaming engine 2700 includes a dedicated 4-dimensional streamaddress generator 2711/2721 that can each generate one new non-alignedrequest per cycle. Address generators 2711/2721 output 512-bit alignedaddresses that overlap the elements in the sequence defined by thestream parameters. This will be further described below.

Each address generator 2711/2711 connects to a dedicated micro tablelook-aside buffer (μTLB) 2712/2722. The μTLB 2712/2722 converts a single48-bit virtual address to a 44-bit physical address each cycle. EachμTLB 2712/2722 has 8 entries, covering a minimum of 32 kB with 4 kBpages or a maximum of 16 MB with 2 MB pages. Each address generator2711/2721 generates 2 addresses per cycle. The μTLB 2712/2722 onlytranslates 1 address per cycle. To maintain throughput, streaming engine2700 takes advantage of the fact that most stream references will bewithin the same 4 kB page. Thus the address translation does not modifybits 0 to 11 of the address. If aout0 and aout1 line in the same 4 kBpage (aout0[47:12] are the same aout1[47:12]), then the μTLB 2712/2722only translates aout0 and reuses the translation for the upper bits ofboth addresses.

Translated addresses are queued in command queue 2713/2723. Theseaddresses are aligned with information from the corresponding StorageAllocation and Tracking block 2714/2724. Streaming engine 2700 does notexplicitly manage μTLB 2712/2722. The system memory management unit(MMU) invalidates μTLBs as necessary during context switches.

Storage Allocation and Tracking 2714/2724 manages the stream's internalstorage, discovering data reuse and tracking the lifetime of each pieceof data. This will be further described below.

Reference queue 2715/2725 stores the sequence of references generated bythe corresponding address generator 2711/2721. This information drivesthe data formatting network so that it can present data to centralprocessing unit core 110 in the correct order. Each entry in referencequeue 2715/2725 contains the information necessary to read data out ofthe data store and align it for central processing unit core 110.Reference queue 2715/2725 maintains the following information listed inTable 5 in each slot:

TABLE 5 Data Slot Low Slot number for the lower half of data associatedwith aout0 Data Slot High Slot number for the upper half of dataassociated with aout1 Rotation Number of bytes to rotate data to alignnext element with lane 0 Length Number of valid bytes in this referenceStorage allocation and tracking 2714/2724 inserts references inreference queue 2715/2725 as address generator 2711/2721 generates newaddresses. Storage allocation and tracking 2714/2724 removes referencesfrom reference queue 2715/2725 when the data becomes available and thereis room in the stream holding registers. As storage allocation andtracking 2714/2724 removes slot references from reference queue2715/2725 and formats data, it checks whether the references representthe last reference to the corresponding slots. Storage allocation andtracking 2714/2724 compares reference queue 2715/2725 removal pointeragainst the slot's recorded Last Reference. If they match, then storageallocation and tracking 2714/2724 marks the slot inactive once it's donewith the data.

Streaming engine 2700 has data storage 2716/2737 for an arbitrary numberof elements. Deep buffering allows the streaming engine to fetch farahead in the stream, hiding memory system latency. The right amount ofbuffering might vary from product generation to generation. In thecurrent preferred embodiment streaming engine 2700 dedicates 32 slots toeach stream. Each slot holds 64 bytes of data.

Butterfly network 2717/2727 consists of a 7 stage butterfly network.Butterfly network 2717/2727 receives 128 bytes of input and generates 64bytes of output. The first stage of the butterfly is actually ahalf-stage. It collects bytes from both slots that match a non-alignedfetch and merges them into a single, rotated 64-byte array. Theremaining 6 stages form a standard butterfly network. Butterfly network2717/2727 performs the following operations: rotates the next elementdown to byte lane 0; promotes data types by one power of 2, ifrequested; swaps real and imaginary components of complex numbers, ifrequested; converts big endian to little endian if central processingunit core 110 is presently in big endian mode. The user specifieselement size, type promotion and real/imaginary swap as part of thestream's parameters.

Streaming engine 2700 attempts to fetch and format data ahead of centralprocessing unit core 110's demand for it, so that it can maintain fullthroughput. Holding registers 2718/2728 provide a small amount ofbuffering so that the process remains fully pipelined. Holding registers2718/2728 are not directly architecturally visible, except for the factthat streaming engine 2700 provides full throughput.

The two streams 2710/2720 share a pair of independent L2 interfaces2730: L2 Interface A (IFA) 2733 and L2 Interface B (IFB) 2734. Each L2interface provides 512 bits/cycle throughput direct to the L2 controllerfor an aggregate bandwidth of 1024 bits/cycle. The L2 interfaces use thecredit-based multicore bus architecture (MBA) protocol. The L2controller assigns each interface its own pool of command credits. Thepool should have sufficient credits so that each interface can sendsufficient requests to achieve full read-return bandwidth when readingL2 RAM, L2 cache and multicore shared memory controller (MSMC) memory(described below).

To maximize performance, both streams can use both L2 interfaces,allowing a single stream to send a peak command rate of 2requests/cycle. Each interface prefers one stream over the other, butthis preference changes dynamically from request to request. IFA 2733and IFB 2734 always prefer opposite streams, when IFA 2733 prefersStream 0, IFB 2734 prefers Stream 1 and vice versa.

Arbiter 2731/2732 ahead of each interface 2733/2734 applies thefollowing basic protocol on every cycle it has credits available.Arbiter 2731/2732 checks if the preferred stream has a command ready tosend. If so, arbiter 2731/2732 chooses that command. Arbiter 2731/2732next checks if an alternate stream has at least two requests ready tosend, or one command and no credits. If so, arbiter 2731/2732 pulls acommand from the alternate stream. If either interface issues a command,the notion of preferred and alternate streams swap for the next request.Using this simple algorithm, the two interfaces dispatch requests asquickly as possible while retaining fairness between the two streams.The first rule ensures that each stream can send a request on everycycle that has available credits. The second rule provides a mechanismfor one stream to borrow the other's interface when the second interfaceis idle. The third rule spreads the bandwidth demand for each streamacross both interfaces, ensuring neither interface becomes a bottleneckby itself.

Coarse Grain Rotator 2735/2736 enables streaming engine 2700 to supporta transposed matrix addressing mode. In this mode, streaming engine 2700interchanges the two innermost dimensions of its multidimensional loop.This accesses an array column-wise rather than row-wise. Rotator2735/2736 is not architecturally visible, except as enabling thistransposed access mode.

The stream definition template provides the full structure of a streamthat contains data. The iteration counts and dimensions provide most ofthe structure, while the various flags provide the rest of the details.For all data-containing streams, the streaming engine defines a singlestream template. All stream types it supports fit this template. Thenumbers above each field indicate byte numbers within a 256-bit vector.The streaming engine defines a four-level loop nest for addressingelements within the stream. Most of the fields in the stream templatemap directly to the parameters in that algorithm. FIG. 28 illustratesstream template register 2800. The numbers above the fields are bytenumbers within a 512-bit vector. Table 6 shows the stream fielddefinitions of a stream template.

TABLE 6 Field Size Name Description Bits ICNT0 Iteration count for loop0 (innermost) 32 ICNT1 Iteration count for loop 1 32 ICNT2 Iterationcount for loop 2 32 ICNT3 Iteration count for loop 3 (outermost) 8 DIM1Signed dimension for loop 1 32 DIM2 Signed dimension for loop 2 32 DIM3Signed dimension for loop 3 32 FLAGS Stream modifier flags 24In the current example DIM0 is always equal to is ELEM_BYTES definingphysically contiguous data. The stream template includes mostly 32-bitfields. The stream template limits ICNT3 to 8 bits and the FLAGS fieldto 24 bits. Streaming engine 2700 interprets all iteration counts asunsigned integers and all dimensions as unscaled signed integers. Thetemplate above fully specifies the type of elements, length anddimensions of the stream. The stream instructions separately specify astart address. This would typically be by specification of a scalarregister in scalar register file 211 which stores this start address.This allows a program to open multiple streams using the same template.

FIG. 29 illustrates sub-field definitions of the flags field 2900. Asshown in FIG. 29 the flags field 2900 is 3 bytes or 24 bits. FIG. 29shows bit numbers of the fields. Table 7 shows the definition of thesefields.

TABLE 7 Size Field Name Description Bits ELTYPE Type of data element 4DIR Stream direction 1 0 forward direction 1 reverse direction TRANSPOSETwo dimensional transpose mode 3 PROMOTE Promotion mode 2 THROTTLE Fetchahead throttle mode 2

The Element Type (ELTYPE) field 2901 defines the data type of theelements in the stream. The coding of the four bits of this field isdefined as shown in Table 8.

TABLE 8 Total Sub-element Element Real - ELTYPE Size Bits Size BitsComplex Bytes/Element 0000 8 8 real 1 0001 16 16 real 2 0010 32 32 real4 0011 64 64 real 8 0100 reserved 0101 reserved 0110 reserved 0111reserved 1000 8 16 complex 2 no swap 1001 16 32 complex 4 no swap 101032 64 complex 8 no swap 1011 64 128 complex 16 no swap 1100 8 16 complex2 swapped 1101 16 32 complex 4 swapped 1110 32 64 complex 8 swapped 111164 128 complex 16 swapped

Sub-Element Size determines the type for purposes of type promotion andvector lane width. For example, 16-bit sub-elements get promoted to32-bit sub-elements when a stream requests type promotion. The vectorlane width matters when central processing unit core 110 operates in bigendian mode, as it always lays out vectors in little endian order.

Total Element Size determines the minimal granularity of the stream. Inthe stream addressing model, it determines the number of bytes thestream fetches for each iteration of the innermost loop. Streams alwaysread whole elements, either in increasing or decreasing order.Therefore, the innermost dimension of a stream spansICNT0×total-element-size bytes.

Real-Complex Type determines whether the streaming engine treats eachelement as a real number or two parts (real/imaginary ormagnitude/angle) of a complex number. This field also specifies whetherto swap the two parts of complex numbers. Complex types have a totalelement size that is twice their sub-element size. Otherwise, thesub-element size equals total element size.

The DIR bit 2902 determines the direction of fetch of the inner loop(Loop0). If the DIR bit 2902 is 0 then Loop0 fetches are in the forwarddirection toward increasing addresses. If the DIR bit 2902 is 1 thenLoop0 fetches are in the backward direction toward decreasing addresses.

The TRANSPOSE field 2903 determines whether the streaming engineaccesses the stream in a transposed order. The transposed orderexchanges the inner two addressing levels. The TRANSPOSE field alsoindicated the granularity it transposes the stream. The coding of thefour bits of this field is defined as shown in Table 9.

TABLE 9 Transpose Stream Advance TRANSPOSE Duplication Granule BytesRate 0000 both disabled 64 bytes 0001 reserved 0010 reserved 0011transpose 4 16 rows 0100 transpose 8 8 rows 0101 transpose 16 4 rows0110 transpose 32 2 rows 0111 reserved 1000 duplicate 1 1 byte 1001duplicate 2 2 bytes 1010 duplicate 4 4 bytes 1011 duplicate 8 8 bytes1100 duplicate 16 16 bytes 1101 duplicate 32 32 bytes 1110 reserved 1111reservedStreaming engine 2700 actually transposes at a different granularitythan the element size. This allows programs to fetch multiple columns ofelements from each row. The transpose granularity must be no smallerthan the element size.

The PROMOTE field 2904 controls whether the streaming engine promotessub-elements in the stream and the type of promotion. When enabled,streaming engine 2700 promotes types by a single power-of-2 size. Thecoding of the two bits of this field is defined as shown in Table 10.

TABLE 10 PROMOTE Description 00 no promotion 01 unsigned integerpromotion, zero extend 10 signed integer promotion, sign extend 11floating point promotion

When the stream specifies No promotion, each sub-element occupies avector lane equal in width to the size specified by ELTYPE. Otherwise,each sub-element occupies a vector lane twice as large. When PROMOTE is00, the streaming engine fetches half as much data from memory tosatisfy the same number of stream fetches.

Promotion modes 01b and 10b treat the incoming sub-elements as unsignedand signed integers, respectively. For unsigned integers, the streamingengine promotes by filling the new bits with zeros. For signed integersthe streaming engine promotes by filling the new bits with copies of thesign bit. Positive signed integers have a most significant bit equal to0. On promotion of positive signed integers, the new bits are zerofilled. Negative signed integers have a most significant bit equal to 1.On promotion of negative signed integers, the new bits are 1 filled.

Promotion mode 11b treats the incoming sub-elements as floating pointnumbers. Floating point promotion treats each sub-element as a floatingpoint type. The streaming engine supports two floating point promotions:short float (16-bit) to single precision float (32-bit); singleprecision float (32-bit) to double precision float (64-bit).

The THROTTLE field 2905 controls how aggressively the streaming enginefetches ahead of central processing unit core 110. The coding of the twobits of this field is defined as shown in Table 11.

TABLE 11 THROTTLE Description 00 Minimum throttling, maximum fetch ahead01 Less throttling, more fetch ahead 10 More throttling, less fetchahead 11 Maximum throttling, minimum fetch aheadTHROTTLE does not change the meaning of the stream, and serves only as ahint. The streaming engine may ignore this field. Programs should notrely on the specific throttle behavior for program correctness, becausethe architecture does not specify the precise throttle behavior.THROTTLE allows programmers to provide hints to the hardware about theprogram's own behavior. By default, the streaming engine attempts to getas far ahead of central processing unit core 110 as it can to hide asmuch latency as possible, while providing full stream throughput tocentral processing unit core 110. While several key applications needthis level of throughput, it can lead to bad system level behavior forothers. For example, the streaming engine discards all fetched dataacross context switches. Therefore, aggressive fetch-ahead can lead towasted bandwidth in a system with large numbers of context switches.Aggressive fetch-ahead only makes sense in those systems if centralprocessing unit core 110 consumes data very quickly.

The central processing unit core 110 exposes the streaming engine toprograms through a small number of instructions and specializedregisters. A STROPEN instruction opens a stream. The STROPEN commandspecifies a stream number indicating opening stream 0 or stream 1. TheSTROPEN specifies a stream template register which stores the streamtemplate as described above. The arguments of the STROPEN instructionare listed in Table 12.

TABLE 12 Argument Description Stream Start Sealer register storingstream start Address Register address Steam Number Stream 0 or Stream 1Stream Template Vector register storing stream Register template dataThe stream start address register is preferably a scalar register ingeneral scalar register file 211. The STROPEN instruction specifiesstream 0 or stream 1 by its opcode. The stream template register ispreferably a vector register in general vector register file 221. If thespecified stream is active the STROPEN instruction closes the priorstream and replaces the stream with the specified stream.

A STRCLOSE instruction closes a stream. The STRCLOSE command specifiesthe stream number of the stream to be closed.

A STRSAVE instruction captures sufficient state information of aspecified stream to restart that stream in the future. A STRRSTRinstruction restores a previously saved stream. A STRSAVE instructiondoes not save any of the data of the stream. A STRSAVE instruction savesonly metadata. The stream re-fetches data in response to a STRRSTRinstruction.

Streaming engine is in one of three states: Inactive; Active; or Frozen.When inactive the streaming engine does nothing. Any attempt to fetchdata from an inactive streaming engine is an error. Until the programopens a stream, the streaming engine is inactive. After the programconsumes all the elements in the stream or the program closes thestream, the streaming engine also becomes inactive. Programs which usestreams explicitly activate and inactivate the streaming engine. Theoperating environment manages streams across context-switch boundariesvia the streaming engine's implicit freeze behavior, coupled with itsown explicit save and restore actions.

Active streaming engines have a stream associated with them. Programscan fetch new stream elements from active streaming engines. Streamingengines remain active until one of the following. When the streamfetches the last element from the stream, it becomes inactive. Whenprogram explicitly closes the stream, it becomes inactive. When centralprocessing unit core 110 responds to an interrupt or exception, thestreaming engine freezes. Frozen streaming engines capture all the statenecessary to resume the stream where it was when the streaming enginefroze. The streaming engines freeze in response to interrupts andexceptions. This combines with special instructions to save and restorethe frozen stream context, so that operating environments can cleanlyswitch contexts. Frozen streams reactivate when central processing unitcore 110 returns to the interrupted context.

FIG. 30 is a partial schematic diagram 3000 illustrating the streaminput operand coding described above. FIG. 30 illustrates decoding src1field 1305 of one instruction of a corresponding src1 input offunctional unit 3020. These same circuits are duplicated for src2/cstfield 1304 and the src2 input of functional unit 3020. In addition,these circuits are duplicated for each instruction within an executepacket that can be dispatched simultaneously.

Instruction decoder 113 receives bits 13 to 17 comprising src1 field1305 of an instruction. The opcode field opcode field (bits 4 to 12 forall instructions and additionally bits 28 to 31 for unconditionalinstructions) unambiguously specifies a corresponding functional unit3020. In this embodiment functional unit 3020 could be L2 unit 241, S2unit 242, M2 unit 243, N2 unit 244 or C unit 245. The relevant part ofinstruction decoder 113 illustrated in FIG. 30 decodes src1 bit field1305. Sub-decoder 3011 determines whether src1 bit field 1305 is in therange from 00000 to 01111. If this is the case, sub-decoder 3011supplies a corresponding register number to global vector register file231. In this example this register field is the four least significantbits of src1 bit field 1305. Global vector register file 231 recallsdata stored in the register corresponding to this register number andsupplies this data to the src1 input of functional unit 3020. Thisdecoding is generally known in the art.

Sub-decoder 3012 determines whether src1 bit field 1305 is in the rangefrom 10000 to 10111. If this is the case, sub-decoder 3012 supplies acorresponding register number to the corresponding local vector registerfile. If the instruction is directed to L2 unit 241 or S2 unit 242, thecorresponding local vector register file is local vector register field232. If the instruction is directed to M2 unit 243, N2 unit 244 or Cunit 245, the corresponding local vector register file is local vectorregister field 233. In this example this register field is the threeleast significant bits of src1 bit field 1305. Local vector registerfile 231 recalls data stored in the register corresponding to thisregister number and supplies this data to the src1 input of functionalunit 3020. The corresponding local vector register file 232/233 recallsdata stored in the register corresponding to this register number andsupplies this data to the src1 input of functional unit 3020. Thisdecoding is generally known in the art.

Sub-decoder 3013 determines whether src1 bit field 1305 is 11100. Ifthis is the case, sub-decoder 3013 supplies a stream 0 read signal tostreaming engine 2700. Streaming engine 2700 then supplies stream 0 datastored in holding register 2718 to the src1 input of functional unit3020.

Sub-decoder 3014 determines whether src1 bit field 1305 is 11101. Ifthis is the case, sub-decoder 3014 supplies a stream 0 read signal tostreaming engine 2700. Streaming engine 2700 then supplies stream 0 datastored in holding register 2718 to the src1 input of functional unit3020. Sub-decoder 3014 also supplies an advance signal to stream 0. Aspreviously described, streaming engine 2700 advances to store the nextsequential data elements of stream 0 in holding register 2718.

Sub-decoder 3015 determines whether src1 bit field 1305 is 11110. Ifthis is the case, sub-decoder 3015 supplies a stream 1 read signal tostreaming engine 2700. Streaming engine 2700 then supplies stream 1 datastored in holding register 2728 to the src1 input of functional unit3020.

Sub-decoder 3016 determines whether src1 bit field 1305 is 11111. Ifthis is the case, sub-decoder 3016 supplies a stream 1 read signal tostreaming engine 2700. Streaming engine 2700 then supplies stream 1 datastored in holding register 2728 to the src1 input of functional unit3020. Sub-decoder 3014 also supplies an advance signal to stream 1. Aspreviously described, streaming engine 2700 advances to store the nextsequential data elements of stream 1 in holding register 2728.

Similar circuits are used to select data supplied to scr2 input offunctional unit 3002 in response to the bit coding of src2/cst field1304. The src2 input of functional unit 3020 may be supplied with aconstant input in a manner described above.

The exact number of instruction bits devoted to operand specificationand the number of data registers and streams are design choices. Thoseskilled in the art would realize that other number selections thatdescribed in the application are feasible. In particular, thespecification of a single global vector register file and omission oflocal vector register files is feasible. This invention employs a bitcoding of an input operand selection field to designate a stream readand another bit coding to designate a stream read and advancing thestream.

FIG. 31 is a partial block diagram illustrating the componentssupporting cache storage of fetched stream data for stream 0. Thecomponents for stream 1 are similar. Data storage 2716 is conceptually adata first-in-first-out (FIFO) buffers. In the preferred embodiment thisFIFO is implemented in a cache fashion as detailed below.

Early address generator 3101 generates the address of a memory fetch.Late address generator 3102 generates the address of the next element(s)to be loaded into the stream head register 2718. Stream data storage2716 buffers stream data between its fetch from memory and supply to thefunctional units of central processing unit core 110. Data movement iscontrolled with regard to data stored in reference queue 2515.

FIG. 32 illustrates the details of plural cache lines used in streamdata storage 2716. Cache structure 3200 includes cache lines 3210, 3220and 3220 representative of the internal structure. Cache lines 3210,3220 and 3230 include: address tags 3211; valid bits 3212; ready bits3213; action bits 3214; least recently used (LRU) indicators 3215;parity bits 3216; and plural data words 3217. The bit length of datawords 3217 is set by the minimal addressable data amount of centralprocessing unit core 110. This is typically 8 bits/1 byte.

Cache structure 3200 stores data from more distant memories which areaccessed by a multi-bit address. Cache structure 3200 is organized tofacilitate this storage and to facilitate finding such data in thecache. Each cache line 3210, 3220 and 3230 typically stores 2^(N)respective data words 3217, where N is an integer. Address tags 3211includes the most significant address bits for the corresponding cacheline 3210, 3220 and 3230. The position of data words 3217 within thecorresponding cache line 3210, 3220 and 3230 along the dimension 3201serves as a proxy for the least significant bits of the correspondingaddress.

To determine if a memory access is to data cached within cache structure3200 (a cache hit), cache structure 3200 compares the address tags 3211for all cache lines to the most significant bits of the memory locationaccessed. Upon a detecting a match, the position within the cache linealong dimension 3201 corresponds to the least significant bits of theaddress permitting identification of the data word accessed.

Table 13 notes the character of the valid, ready, active and lastreference fields.

TABLE 13 Valid Single bit indicating whether the tag address is validReady Single bit indicating the data has arrived for this address ActiveSingle bit indicating whether there are any references outstanding tothis data Last Reference Value indicating the most recent reference tothis slot in the reference queue

Each cache line 3210, 3220 and 3230 includes a corresponding valid bit3212. A first state of valid bit 3212 indicates the corresponding datawords 3217 of that cache line are valid. An opposite state of this validbit indicates the corresponding data words 3217 are not valid. There areseveral instances where data stored within cache structure 3200 wouldnot be valid. Upon initial activation of digital signal processor system100 cache structure 3200 would not be loaded. Thus cache structure 3200would not store valid data. Accordingly, all cache lines are initiallymarked invalid. During a cache access a comparison of a requestedaddress with address tags 3211 would not detect a match unless thecorresponding valid bit 3212 indicated the data was valid.

Each cache line 3210, 3220 and 3230 includes a corresponding ready bit3213. A first state of ready bit 3213 indicates that no data for thecorresponding cache line has arrived. A second state of the ready bit3213 indicates that fetched data for this cache line has arrived.

Each cache line 3210, 3220 and 3230 includes a corresponding active bit3214. A first state of active bit 3214 indicates that no reference todata for the corresponding cache line is pending. A second state of theactive bit 3214 indicates that no reference to data for this cache lineis pending.

Each cache line 3210, 3220 and 3230 includes a corresponding lastreference field 3215. Last reference field 3215 typically is pluralbits. Last reference field 3215 indicates the most recent reference tothis slot in the reference queue. As described in further detail below,this information is used for allocation of cache lines.

Each cache line 3210, 3220 and 3230 includes corresponding parity bits3216. The generation and use of parity bit 3216 will be described below.

Cache structure 3200 includes no corresponding dirty bits. According tothe structure of the data stream, data stored in cache structure 3200cannot be changed. Cache structure 3200 is used only for read accesses.Data stored within cache structure 3200 is thus never dirty.Accordingly, there is no need for dirty bits.

Table 14 details the interaction of the valid, ready and active bits.

TABLE 14 Available for Valid Ready Active Interpretation Allocation 0 —— Address invalid Yes 1 0 0 Invalid, cannot have — data pending withoutreference in flight 1 0 1 Request sent for slot, No data pending 1 1 0No active references in Yes flight 1 1 1 Reference in flight, No dataavailableUsing this metadata, the storage allocation and tracking 2714/2724 canidentify data reuse opportunities in the stream. Storage allocation andtracking 2714/2724 performs the following steps for each address. Itcompares the address against the relevant tags in its tag array. On ahit, it cancels the command associated with this address. On a miss, itallocates a free slot, setting Valid=1, Ready=0 and updates the outgoingcommand to direct the data it is fetching to this slot. In either case,a slot number is associated with the address. Storage allocation andtracking 2714/2724 inserts the reference in the reference queue. Storageallocation and tracking 2714/2724 sets Active=1 and updates LastReference to the position of the reference in the reference queue. Thisis the value of the reference queue's insertion pointer at the time ofinsertion. This process converts the generated addresses into the slotnumbers that represent the data. From this point forward, the streamingengine need not track addresses directly.

To maximize reuse and minimize stalls, streaming engine 2700 allocatesslots in the following order: the slot one after the most recentallocation if available in FIFO order; the lowest number available slot,if any; and if no slot available, stall and iterate these two stepsuntil allocation succeeds. This will tend to allocate slots in FIFOorder, but avoids stalling if a particular reuse pattern works againstthat order.

FIG. 33 illustrates a partial schematic view of an early addressgenerator 3101. Early address generator 3102 forms an address forfetching a next element in the defined stream of the correspondingstreaming engine. Start address register 3201 stores a start address ofthe data stream. As previously described, start address register 3301 ispreferably a scalar register in global scalar register file 211designated by the STROPEN instruction that opened the correspondingstream. As known in the art, this start address may be copied from thespecified scalar register and stored locally at the correspondingaddress generator 2711 OR 2721. A first loop of the stream employs Loop0count register 3311, adder 3312, multiplier 3313 and comparator 3314.Loop0 count register 3311 stores the working copy of the iteration countof the first loop (Loop0). For each iteration of Loop0 adder 3312, astriggered by the Next Address signal, adds 1 to the loop count, which isstored back in Loop0 count register 3311. Multiplier 3313 multiplies thecurrent loop count and the quantity ELEM_BYTES. ELEM_BYTES is the sizeof each data element in the loop in bytes. Loop0 traverses data elementsphysically contiguous in memory of the iteration step size isELEM_BYTES.

Comparator 3314 compares the count stored in Loop0 count register 3311(after incrementing by adder 3313) with the value of ICNT0 2810 from thecorresponding stream template register 2800. When the output of adder3312 equals the value of ICNT0 2810 of the stream template register2800, an iteration of Loop0 is complete. Comparator 3314 generates anactive Loop0 End signal. Loop0 count register 3311 is reset to 0 and aniteration of the next higher loop, in this case Loop1, is triggered.

Circuits for the higher loops (Loop1, Loop2, Loop3) are similar to thatillustrated in FIG. 33. Each loop includes a corresponding working loopcount register, adder, multiplier and comparator. The adder of each loopis triggered by the loop end signal of the prior loop. The second inputto each multiplier is the corresponding dimension DIM1, DIM2 and DIMS ofthe corresponding stream template. The comparator of each loop comparesthe working loop register count with the corresponding iteration valueICNT1, ICTN2 and ICTN3 of the corresponding stream template register2800. A loop end signal generates an iteration of the next higher loop.A loop end signal from Loop3 ends the stream.

FIG. 33 illustrates the generation of early loop0 count. Early loop0count equals the updated data stored in the corresponding working countregister 3311. Early loop0 count is updated on each change of workingloop0 count register 3311. The early loop counts for the higher loops(Loop1, Loop2, Loop3) are similarly generated.

FIG. 33 illustrates the generation of early loop0 address. Early loop0address equals the data output from multiplier 3313. Early loop0 addressis updated on each change of working loop0 count register 3311. Similarcircuits for Loop1, Loop2 and Loop3 produce corresponding early loopaddresses.

FIGS. 34A and 34B are together a flow chart 3400 illustrating the mannerof data fetch for a stream employing the early address generator 3101.Flow chart 3400 begins at entry point A to test block 3401. Test block3401 determines whether a next fetch from memory is required. Thepreferred manner of determining this is detailed below. If test block3201 determines no memory fetch is required (No at test block 3201), theflow loops back to test block 3401 for another determination. Flowremains at test block 3401 until a memory fetch is required (Yes at testblock 3401).

When a memory fetch is required (Yes at test block 3401), block 3402performs a tag compare. The most significant bits of the next fetchaddress are compared with the address tags 3211 of each of the cachelines in stream data storage 2716. As previously described a match isnot confirmed unless the corresponding valid bit 3212 indicates valid.

Test block 3403 determines if the address compare of block 3402 found amatch (hit). If test block 3403 determined a match (hit) (Yes at testblock 3402), then flow advances to block 3404. Block 3404 associates thecurrent fetch address with the cache line/slot number generating thehit. Upon a hit, the stream data for the current fetch address isalready stored in stream data storage 2717. Block 3155 updates thereference queue. The corresponding reference queue 2715 or 2725 storesdata indicating the location of data within the cache. As previouslydescribed this data stored in the reference queue permits recall andalignment with the vector in the corresponding head register 2718/2728.

Block 3406 generates the next address in the stream. This will generallybe the next element in loop0. However the current stream element mayhave been the end of one of the nested loops. The next address isgenerated according to the specified stream data pattern in a mannerpreviously described. Flow returns to test block 3401 via entry point A.

In the case of a miss (No at test block 3403), the data corresponding tothe current fetch address is not stored in the corresponding stream datastorage 2616 or 2726. The process 3400 attempts to allocate a new cacheline to the current fetch address. Test block 3407 determines if a cacheline is available for allocation. As will be described below, an unusedcache line may not be available for allocation. Once allocated a cacheline is only deallocated by consumption of all stream data elementsstored in that cache line via central processing unit core 110 readsfrom the corresponding head register 2718/2728. Due to the nature of thedata stream, all previously allocated cache lines store data to be usedby central processing unit core 110 before data corresponding to thecurrent fetch address. Thus cache lines are not evicted because theystore data needed before the current data. Accordingly, a cache linecannot be allocated until all the stream data stored in that cache lineis consumed by central processing unit core 110. As detailed below suchdata consumption is a necessary but not sufficient condition for thecache line to be re-allocated. If no cache line is available forallocation (No at test block 3407), block 3408 stalls the flow for onememory cycle. Process remains in this loop until at least one cache lineis available for allocation (Yes at test block 3407).

If at least one cache line is available for allocation (Yes at testblock 3407), then test block 3409 determines if the cache line followingthe last used cache line is available for allocation. If this cache lineis available for allocation (Yes at test block 3409), then block 3410allocates this cache line. If this cache line is not available forallocation (No at test block 3409), then block 3411 allocates the lowestcache line available. The preferred pattern of allocation is infirst-in-first-out (FIFO) order. Thus the preferred cache line toallocate is the cache line following the last used cache line. Thus theactive data would tend to rotate within the cache lines with allocationfollowing consumption by central processing unit core 110. Thisallocation pattern tends to allocate slots in FIFO order. The allocationpattern described above (allocating the lowest cache line available ifthe cache line following the last use cache line is unavailable) avoidsstalling if a particular data reuse pattern works against this FIFOorder.

Upon allocation of a cache line for the current fetch address, flowadvances to block 3412. Block 3412 sets the valid bit to 1, the readybit to 0 and the active bit to 1 of the just allocate cache line. Thisindicates that the just allocated cache line is valid, not ready anddata is in flight to the cache line. Next block 3413 requests cacheservice for the just allocated cache line. In the preferred embodimentof this invention stream data bypasses L1D cache 123 and requests thedata directly from L2 cache 130. This design choice prevents stream datafrom interfering with other data used by central processing unit core110.

Test block 3414 waits for the data to be returned to stream data storage2716. The delay for return of the data is unknown and variable. Thisnewly fetched data may be stored in L2 cache 130 and available quickly.This data might not be stored in L2 cache 130 and thus must be fetchedfrom another level of memory. This other level of memory may be sharedbetween several requesters. In this event there is generally anarbitration between requesters for access. Accordingly the memorylatency is both unknown and variable.

Upon return of the requested data (Yes at test block 3414), block 3415stores this data in the allocated cache line. Block 3416 changes thecorresponding ready bit to 1, indicating the data is ready forconsumption.

Process flow advances to block 3404. Block 3404 associates the currentfetch address with the cache line/slot number generating the hit. Block3405 updates the reference queue. The corresponding reference queue 2715or 2725 stores data indicating the location of data within the cache.Block 3406 generates the next address in the stream. Flow returns totest block 3401 via entry point A.

FIG. 35 is similar to FIG. 33, illustrating exemplary circuitsgenerating the late Loop0 count and late Loop0 address. A first loop ofthe stream employs Loop0 count register 3511, adder 3512, multiplier3513 and comparator 3514. Multiplier 3513 has a second input receivingthe ELEM_BYTES. In FIG. 35 each iteration is trigged by a stream advancesignal. Thus the count values of the late count circuits and the addressvalues of the late addresses reflect the next data to be supplied to thefunctional units. This would be the stream elements stored incorresponding head registers 2218/2228. Circuits for loops Loop1, Loop2and Loop3 are similar except the second input to the correspondingmultipliers are the corresponding dimension DIM1, DIM2 and DIMS of thecorresponding stream template. A loop end signal generates an iterationof the next higher loop.

As described each streaming engine keeps two copies of the loop countsand loop addresses: an early copy which indicates where the streamingengine is currently fetching; and a late copy, which indicates the lastiteration central processing unit core 110 has committed. In most cases,the late copy gives the clearest indication of the current state of theprogram. The early copy just indicates of how far the streaming enginehas fetched ahead of central processing unit core 110. The differencebetween the early copies and the late copies reflects the amount ofstream data stored in data storage 2217/2237. The count data and theaddress data are related. Any particular embodiment may include onlycount data, only address data or both.

It is known in the art that any count such as illustrated in FIGS. 33and 35 may be embodied as count down or count up. FIGS. 33 and 35illustrate count up embodiments. Those skilled in the art wouldunderstand how to modify intialization and comparisons to embody thisinvention in count down circuits. Additionally, those skilled in the artwould understand how to modify the comparisons to embody the addressingmodes described.

FIG. 36 is a flow chart 3600 illustrating the manner of filing thestream head register employing late address generator 3102. Flow chart3600 begins at test block 3601. Test block 3601 determines whether anext data element of the data stream is to be loaded into thecorresponding stream head register. As previously described, in thepreferred embodiment stream data remains in the stream head registerwhere it can be fetched more than once by plural functional units, untilthe streaming engine encounters a read and advance operand coding. Upondetecting a read and advance operand coding, the streaming enginesupplies the current data in the head register to the requestingfunctional unit and replaces the data in the stream head register withthe next sequential data in the data stream. As previously noted thestreaming engine may fill the head register with more than one streamdata element depending on the relationship of the specified stream datasize to the vector size. If the stream head register data is not to bereplaced (No at test block 3601), the control loops back to test block3601 for another determination. Flow remains at test block 3601 until areplacement of data stored in the head register is required (Yes at testblock 3601).

When a replacement of the head register is required (Yes at test block3601), block 3602 obtains the address of data for the next head registerfill. In the preferred embodiment this determination is made via thecorresponding reference queue 2715/2725. Text block 3603 determineswhether this just accessed cache line is valid. If the cache line isvalid (Yes at text block 3603), then block 3604 recalls the data to atemporary buffer. If the cache line is not valid (No at text block3603), then block 3605 zero fills the temporary buffer.

In either case block 3606 rotates and formats the data. This process hasbeen described above. Data rotation aligns the data elements of thestream in the proper slots in the head register. Date formattingincludes complex number sub-element swapping, data promotion,transposition and duplication as previously described. Followingrotation and formatting, block 3607 stores the data in the correspondinghead register 2718/2728 in vector slots as previously described.

FIG. 37 illustrates an exemplary bit assignment of an extended controlregister (ECR) denoted SEn_ICNT 3700. According to the preferredembodiment streaming engine 2700 includes two such ECRs, SE0_INCT forstream 0 and SE1_INCT for stream 1. Each SEn_ICNT stores early and lateloop counts of the loops defined in the corresponding stream templatefor each open stream. SEn_ICNT 3700 allows a programmer to determinewhere the streaming engine is within a given stream while debugging aprogram. In the preferred embodiment SEn_ICNT 3700 is accessible only bythe debugger. Any other attempt to access SEn_ICNT 3700 generates aprivilege error. SEn_ICNT 3700 includes early loop0 count 3701, earlyloop1 count 3702, early loop2 count 3703, early loop3 count 3704, lateloop0 count 3705, late loop1 count 3706, late loop2 count 3707 and lateloop3 count 3708. Each count value field has a bit width at least asgreat as the bit width of the corresponding iteration count of thecorresponding stream template register. The respective early and latecounts are similar but updated at different times that will be furtherdescribed below.

FIG. 38 illustrates an exemplary bit assignment of an ECR denotedSEn_ADDR 3800. According to the preferred embodiment streaming engine2700 includes two such ECRs, SE0_ADDR for stream 0 and SE1_ADDR forstream 1. Each SEn_ADDR stores early and late addresses of the loopsdefined in the corresponding stream template for each open stream.SEn_ADDR 3800 allows a programmer to determine where the streamingengine is within a given stream while debugging a program. In thepreferred embodiment SEn_ADDR 3800 is accessible only by the debugger.Any other attempt to access SEn_ADDR 3800 generates a privilege error.SEn_ADDR 3800 includes early loop0 address 3801, early loop1 address3802, early loop2 address 3803, early loop3 address 3804, late loop0address 3805, late loop1 address 3806, late loop2 address 3807 and lateloop3 address 3808. Each address field has a bit width at least as greatas the bit width of the addresses of processor 100. The respective earlyand late addresses are similar but updated at different times that willbe further described below.

Streaming engine 2700 reports a range of faults including: programmingerrors; memory translation faults; bit errors detected in the storagearray; bus errors reported by the system; and functional failures.Streaming engine reports faults synchronously, aligned to programexecution.

Streaming engine 2700 reports faults synchronously by marking data fromthe streaming engine with a flag indicating a fault. This flagged datatriggers an internal central processing unit core 110 exception event ifand only if the program attempts to consume that data. Any instructionthat attempts to consume the data marked as errant generates a centralprocessing unit core 110 internal exception event. The relationshipbetween errors recognized by streaming engine 2700 and the instructionsthat experience an exception does not depend on stall timing.

When streaming engine 2700 detects a fault to report, it marks theaffected data in internal storage 2716/2726. In parallel, streamingengine 2700 records the details of the error in a corresponding pair ofextended control registers, fault address register SEn_FAR and faultsource register SEn_FSR. Each streaming has its own set of extendedcontrol registers SEn_FAR and SEn_FSR. Extended control registersSE0_FAR and SE0_FSR store data for stream 0 and extended controlregisters SE1_FAR and SE1_FSR store data for stream 1. Depending uponthe error, streaming engine 2700 may also disable further requestgeneration for the stream. In the preferred embodiment fault addressregister SEn_FAR and fault source register SEn_FSR are accessible onlyby the debugger.

Streaming engine 2700 records the virtual address that triggered theerror in the corresponding extended control register SEn_FAR. Anexemplary SEn_FAR register 3900 is illustrated in FIG. 39. In thecurrent embodiment each SEn_FAR register is 64 bits. A first field 3901stores the virtual address of the streaming engine fetch generating thefault. In the preferred embodiment this virtual address is 49 bits (bits0 to 40). The remaining bits of SEn_FAR register (second field 3902) arereserved and unused in the current example. These bits may be used ifthe virtual address of an embodiment exceeds 49 bits.

Streaming engine 270 records a fault status in a corresponding registerSEn_FSR. An exemplary SEn_FSR register 4000 is illustrated in FIG. 40.In the current embodiment each SEn_FSR register is 64 bits. A firstfield 4001 stores the details of the encounter fault. A second field4002 (the remaining bits of SEn_FSR register 4000) are reserved andunused in the current example embodiment. Table 15 shows the coding ofdetails field 4001 in SEn_FSR register 4000.

TABLE 15 Error Code Bit Field 4001 Error 0000000000000 No error11xxxxxxxxxxx Memory management unit error 1010000000xxx Level 2 cacheerror 1000000000000 Reserved 1000000000001 Streaming Engine: internalstorage parity error 1000000000010 Streaming Engine: invalid streamtemplate 1000000000011 Streaming Engine: stream active mismatch1000000000100 Streaming Engine: CPU spurious read while stream inactive1000000000101 Streaming Engine: address bit 48 rollover 1000000000110Streaming Engine: Level 2 cache fragmented data return 1000000000111ReservedAs shown in Table 15, bit 12 the most significant bit of bit field 4001is 0 if there is no error and 1 if there is an error. A set of codingsof bit field 4001 are set aside to indicate memory management uniterrors. Memory management unit errors include memory translation faultssuch as: lack of privilege: lack of a valid translation entry for thepresented virtual address; and an actual physical error encounteredduring translation. A second set of codings of bit field 4001 are setaside to indicate level 2 cache errors. Other codings indicate streamingengine errors.

A coding of “1000000000001” indicates a streaming engine internalstorage data error. This indicates that the corresponding data storage2716/2726 has an error. In the preferred embodiment parity bits arestored with the data bits. Upon use a new parity is computed from thedata and compared with the stored parity. A difference between the newparity and the stored parity indicates an internal storage error.

A coding of “1000000000010” indicates a streaming engine stream templateerror. Upon opening a stream central processing unit core 110 indicatesa stream template register storing the parameters for the data stream.Some feasible data codings of the stream template register (FIGS. 28 and29) are produce combinations of options not supported by the preferredembodiment. These include: an element size after promotion and elementduplication the exceeds the vector length; a transpose granule sizeafter promotion, element duplication and decimation exceeds the vectorlength; invalid dimension or address alignment for transposed streams;enabling decimation while promotion is disabled; out of range iterationcount for transposed stream; reserved encoding used in any field; andnon-zero values in any reserved field. The latter two errors areindications of an improperly specified template.

A coding of “1000000000011” indicates a streaming engine activemismatch. This would occur when central processing unit core 110attempts an operation not consistent with the current stream state(inactive, active, frozen).

A coding of “1000000000100” indicates a central processing unit core 110spurious read while the stream is inactive. An inactive stream cannot beread. An attempt to read an inactive stream generates this error.

A coding of “1000000000101” indicates an address bit 48 rollover.Thought the virtual address has 49 bits it is generally stored in a 64bit scalar register. Central processing unit core 110 sign extends the49 bit virtual address to 64 bits for storage. Accordingly, change ofbit 49 (the sign bit) of the virtual address is not permitted. Anystream data fetch operation toggling bit 49 of the virtual addressgenerates this error.

A coding of “1000000000110” indicates a Level 2 cache fragmented datareturn. A stream data fetch that results in a fragmented data returnfrom combined level 2 cache 130 generates this error.

The coding is “1000000000111” reserved and not used.

Upon detection of any of these streaming engine faults, the streamingengine attempts to store the virtual address of the data fetchgenerating the error in SEn_FAR and stores a corresponding error code inSEn_FSR as detailed above. The streaming engine is not always able toassociate an address with every fault. In the preferred embodiment, thestreaming engine is unable to report an error address in thesecircumstances: invalid stream state when activating frozen stream (badSTRRSTR); bit error in the data storage array during a transposedstream; bus error reported by the system during a transposed stream. Inthose circumstances, the streaming engine will clear SEn_FAR to 0.Because the streaming engine can generate multiple addresses in parallelto satisfy a single data-phase destined for central processing unit core110, the streaming engines chooses one of those addresses to report ifboth addresses experience a fault. The streaming engine does not definean architectural order among these parallel addresses; rather, theaddress the streaming engine chooses to report the fault for when twofaults occur in parallel is implementation defined.

Upon occurrence of these errors, the streaming engine does notimmediately trigger an error to central processing unit core 110. Thefault status flows with the affected data through the streaming enginedata storage 2716/2726 as data is consumed by central processing unitcore 110. Central processing unit core 110 triggers an internalexception event if and when the program actually consumes the data. Dueto pipelined nature of central processing unit core 110, many cycles maypass between when the streaming engine detects a fault, and when centralprocessing unit core 110 takes an exception. If the program neverconsumes the affected data, central processing unit core 110 will nevergenerate an exception event. For example, if a program opens a streamwith an invalid template and then closes this stream without reading anydata from it, the streaming engine will not cause central processingunit core 110 to take an exception. This behavior treats streamingengine fetches as speculative until the program running on centralprocessing unit core 110 actually consumes the data. If the fault isrecoverable, such as a page fault, then the stream will resume at thepoint of the data item that triggered the exception when centralprocessing unit core 110 returns from the exception event handler.

FIG. 41 illustrates error detection and correction of this invention.Parts illustrated in FIGS. 1 and 27 are given the same referencenumbers. FIG. 41 illustrates connections between central processing unitcore 110, level one instruction cache (L1I) 121, level one data cache(L1D) 123 and level two combined instruction/data cache (L2) 130.

FIG. 41 illustrates Program Memory Controller (PMC) 4110 including L1Icache 121 and error correcting hardware. L1I cache 121 receives datafrom L2 SRAM/cache 130 via 2×256 bit correction unit 4141 and 16×32 bitparity generator 4142. On supply of instructions to central processingunit core 110 the parity bits stored in L1I cache 121 are compared withnewly calculated parity bits from 16×32 bit parity detector 4111. Ifthey match the instructions are supplied to central processing unit core110 via multiplexer 4112. If they do not match, the instructions arerecalled from L2 SRAM/cache 130, then subject to the parity test again.On a cache miss in L1I cache 121, L2 SRAM/cache supplies the requestedinstructions to L1I cache 121 via 2×256 bit correction unit 4141 and16×32 bit parity generator 4142 and in parallel supplies instructionsdirectly to via multiplexer 4122.

FIG. 41 illustrates Data Memory Controller (DMC) 4120 including L1Dcache 123 and error correcting hardware. L1D cache 123 receives datafrom L2 SRAM/cache via 2×256 bit correction unit 4121 and 16×32 bitparity generator 4122. On supply of data to central processing unit core110 the parity bits stored in L1D cache 123 are compared with newlycalculated parity bits from 16×32 bit parity detector 4123. If theymatch the data is supplied to central processing unit core 110 viamultiplexer 4124. If they do not match, the data is recalled from L2SRAM/cache 130, then subject to the parity test again. On a cache missin L1D cache 123, L2 SRAM/cache supplies the requested data to L1D cache123 via 2×256 bit correction unit 4121 and 16×32 bit parity generator4122 and in parallel supplies data directly to central processing unitcore 110 via multiplexer 4124.

Writes from central processing unit core 110 are subject to paritygeneration in 16×32 bit syndrome generator 4125. The data received fromcentral processing unit core 110 and the calculated parity bits arestored in L1D cache 123.

On write back from L1D cache 123 newly calculate parity bits and thestored parity are compared in 2×256 bit syndrome generator 4143. Ifthese match, the data is stored in L2 SRAM/cache 130. If they do notmatch, 2×256 bit syndrome generator 4145 attempts correction. If thecorrection is achieved, the corrected data is stored in L2 SRAM/cache130. Failure of correction generates a fault.

FIG. 41 illustrates stream buffer 125 including separate stream 0 2710and stream 1 2720. Stream 0 2710 receives data from L2 SRAM/cache via2×256 bit correction unit 4151 and 16×32 bit parity generator 4152. Onsupply of data to central processing unit core 110 the parity bitsstored in stream 2210 are compared with newly calculated parity bits in16×32 bit parity detector 4131. If they match the data is supplied tocentral processing unit core 110. If they do not match, there is afault. Stream 1 2720 operates similarly with 2×256 bit correction unit4153, 16×32 bit parity generator 4154 and 16×32 bit parity detector4132.

FIG. 41 illustrates Unified Memory Controller (UMC) 4140 including L2SRAM/cache 130 and error correcting hardware. L2 SRAM/cache 130 receivesdata from multicore shared memory controller (MSMC) 4160 via 2×256 bitsyndrome generator 4144. New parity is generated for storage in L2SRAM/cache 130 and correction is attempted if needed. Upon a non-matchand failure of correction, the data is recalled from MSMC 4160, thensubject to the parity test again. There are no parity checks orcorrection on writes from L2 SRAM/cache 130 to multicore shared memorycontroller (MSMC) 4160.

Periodically 2×256 bit syndrome generation 4145 and 2×256 correction4146 walks through the data stored in L2 SRAM/cache 130. The data andparity is recalled, new parity generated and checked and correctionattempted if needed. If the data is correct, there is no change made inL2 SRAM/cache 130. If data is corrected, the corrected data is storedback in L2 SRAM/cache 130. Failure of data correction generates a fault.

High-performance computing systems, such as the above describedstreaming engines, need low-latency, high-bandwidth access to randomaccess memory pools to properly utilize the available hardware. Memoryperformance scaling has not kept up with computer architecture andprocess enhancements leading to memory bottlenecks in many applications.These state-of-the-art memories, especially off-chip memories, requirespatial and temporal locality to efficiently utilize the data bus andaccess to the data arrays. In multi-core systems, fast memory bandwidthresource tends to be scarce and many compute elements must share accessto it. The arbiters providing this resource allocation will typicallyattempt to provide a fair distribution of the resource for similarpriority traffic. This fairness can actually hinder memory efficiency bydestroying temporal and spatial locality required to efficiently utilizethe memory.

As an example of such a system masters A and B are attempting to accessa memory Z. As greedy requestors, A and B will send as many requests aspossible as fast as possible until they reserve all available databuffering capacity. Then they will stall until some of the previousrequests return. In most systems with large, off-chip memories thememory return bandwidth is lower than the requestors issuing capacity.Thus data requests return slower than originally sent. For a greedymaster, a new request goes out as soon as one returns. Once the databuffering is full, requests issue out and return at the speed of thememory return bandwidth leaving time gaps between requests. This resultsin an initial flurry of back-to-back requests until the data bufferingis full, followed by a steady state with slow, one-by-one tricklingrequests matched to the memories return bandwidth.

This drop in the request rate from A and B requestors reduces thetemporal and spatial locality of each request. This in turn reduces theefficiency of the memory access. This reduced memory efficiency furtherdrops the request return rate causing the time gap between requests togrow, further reducing locality. This feedback relationship degradessystem performance dramatically, especially as the more requestors enterthe pool for access to memory Z.

The greedy master will utilize a request algorithm based on the numberof requests needing to be sent (N) based on the program or function ofthe master and the number of requests the master is capable of sending(M) based on buffering, availability or other constraint. This algorithmtypically looks something like:

1. If N==0, send no requests (none needed).

2. (N>0) and (N≤M), send N requests.

3. (N>0) and (N>M), send M requests.

As previously described, these greedy masters reduce the temporal andspatial locality of requests handled by the memory. This will generallyresult in more memory page changes and increased latency needed forswapping address translation entries. This invention introduceshysteresis in a greedy master request rate. This hysteresis adds ahysteresis command number (R) to the calculation and looks somethinglike this:

1. If N==0, send no requests (none needed).

2. If (N>0) and (N≤M), send N requests.

3. If (N>0) and (N>M):

-   -   a. If (M<R), wait until more buffering is available;    -   b. If (M≥R), issue R requests.        In the second case (N≤M), there is more buffering available than        requests to send. Thus all the pending requests should be sent.        In the third case (N>M), there are more requests than available        buffering. In this case, this invention uses a hysteresis. This        hysteresis does not send any requests until a cluster of R        requests can be sent together. This ensures that the requestor        will cluster R requests together whenever there are more than R        requests to send. This forced clustering potentially increases        the latency on any individual command. However, this forced        clustering ensures better temporal and spatial locality at the        memory to maximize that efficiency. Since the overall system is        bottlenecked on memory bandwidth, maximizing that utilization        will result in an overall higher performing system.

FIG. 42 illustrates a multiprocessor, multi-memory system 4200 to whichthe fetch hysteresis of this invention is applicable. System 4200includes plural data processors. These include data processors 4201 and4202, each consisting of a processor 100 as previously described. Eachprocessor 4201 and 4202 includes a scalar data path 115, a vector datapath 116, a level one instruction cache (L1I) 121, a level one datacache (L1D) 123, a streaming engine 125 and a level two combinedinstruction/data cache (L2) 130. These elements are connected andfunction as previously described. As indicated in FIG. 42, system 4200may include additional data processors of the same type. System 4200includes plural reduced instruction set computer (RISC) centralprocessing units (CPUs) 4211 to 4213. These data processor types merelyserve as examples of this invention and other data processors types ofgreater or lesser numbers are feasible.

Multiprocessor, multi-memory system 4200 includes plural memories ofplural types. These include shared memory type A 4231 and 4232, andshared memory type B 4241 and 4342. System 4200 also includes sharedresource 4251. Shared resource 4251 may include an input device, anoutput device or other type of device shared among the data processors4201, 4202, 4211 and 4213. These memory types are mere examples andother memories and shared resource types of greater or lesser numbersare feasible.

Multicore shared memory controller 4220 mediates data movement betweendata processors 4201, 4202, 4211 . . . 4213 and resources 4231, 4232,4241, 4342 . . . 4251. This mediation includes priority and sequencing,address translation and matching the data flow types of the variousprocessors and memories.

The hysteresis of this portion of the invention assumes that accesslatency to at least some of the shared memories is enhanced when theaccesses are temporally and spatially local rather than uncorrelated.For example, memories that support page mode accesses show reducedlatency when accesses to the same page are adjacent in time. Memoryaccess from a single source have a greater probability of exhibitingthis temporal and spatial locality than temporally adjacent accessesfrom differing sources. Thus a technique that groups accesses from asingle source for supply to a shared resource may reduce latency.

FIGS. 43 and 44 illustrate application of this hysteresis to thestreaming engine of this application. FIG. 43 illustrates a partialschematic diagram of the hardware practicing this invention. FIG. 44illustrates the process of this aspect of the invention. Thisdescription assumes that the burst grouping happens between thestreaming engine and the corresponding level two combined cache. In thiscase the level two combined cache will generally preserve burstiness onseeking service from a higher level memory. Those skilled in the artwould realize that the hysteresis of this invention can be enforced atother locations within the multiprocessor system. For example, multicoreshared memory controller 4220 could provide hysteresis for memoryrequests from RISC CPUs 4211 and 4213.

FIG. 43 illustrates computation of the number or pending requests N, theamount of buffering available M and the hysteresis value R. The numberof pending requests N is computed using a plurality of multipliers 4301,4301, 4303 and 4304. The first multiplier 4301 receives the quantityELEM_BYTES as a first input. This is the size of the data elements ofthe defined data stream. As noted above, ELEM_BYTES is defined by thestream definition template. In the preferred embodiment the streamdefinition template includes a field EFLAGS, a subset of which is theELTYPE field. As shown in Table 8, the ELTYPE field encodes the quantityELEM_BYTES. This encoding is a mere example, those skilled in the artwould recognize other feasible techniques to set the quantityELEM_BYTES. The second input to multiplier 4301 is the quantity Loop0Count from the early count register 3311 illustrated in FIG. 33.Multiplier 4302 multiplies the product of multiplier 4301 by thequantity Loop1 count from early count register 3321 (not illustrated inFIG. 33, but analogous to Loop0 Count register 3311). Similarly,multiplier 4303 multiplies the product of multiplier 4302 by thequantity Loop2 Count from early count register 3331 and multiplier 4304multiplies the product of multiplier 4303 by the quantity Loop3 Countfrom early count register 3341. Since the early loop count registers3311, 3321, 3331 and 3341 store the remaining number of theircorresponding loops, this quantity is the number of data element fetchesrequired by the specified data stream. Multiplying by ELEM_BYTES thusyields the amount of data remaining to be fetched by the data stream.This quantity is designated N.

Block 4311 generates the number of cache lines within the correspondingdata storage 2716/2726 that are unallocated. Multiplier 4312 multiplesthis product by the cache line size. This cache line size must beexpressed in the same units as ELEM_BYTES. The product output ofmultiplier 4312 is the amount of available data buffering M. Uponinitial opening of a data stream no cache lines would be allocated andthe amount of available data buffering M would be at its maximum value.The amount of available data buffering M decreases upon allocation of acache line to new data and increases upon deallocation of a cache lineupon consumption of data by central processing unit core 110.

Register 4321 stores the hysteresis constant R. This is preferably amemory mapped register that can be written to by central processing unitcore 110. Data in register 4321 is read for the fetch hysteresis of thisinvention.

Fetch controller 4331 forms the fetch determinations from the N, M and Rinputs. This fetch determination is made as previously described and asshown in FIG. 44. This fetch determination is used in test block 3401 ofFIG. 34 to determine whether to issue a fetch request from the streamingengine. Fetch controller 4331 counts the number of fetches issued andcontrols test block 3401 fetch issue accordingly. Fetch controller 4330may be implemented in and asynchronous logic machine, as a finite statemachine or as a properly programmed logic controller.

FIG. 44 is a flow chart illustrating processing 4400 of the fetchhysteresis. Test block 4401 determines whether at least one fetchrequest is ready. In the streaming engine embodiment this determineswhether a stream is open that has not fetched and stored all the dataelements. No fetches are required for closed streams. No fetches arerequired if all specified data elements have already been fetched andstored. If no request is ready (No at test block 4401), then test block4401 repeats until at least one request is ready.

If at least one request is ready (Yes at test block 4401), then block4402 compares the number of ready requests N to the amount of databuffering M currently available. In the streaming engine embodiment thenumber of ready requests R equals the number of not yet fetched dataelements in the corresponding data stream multiplied by the data elementsize. In the streaming engine embodiment the amount of buffering Mcurrently available equals the number of unallocated cache linesmultiplied by the cache line size. If the number of ready requests R isless than or equal to the amount of data buffering M currently available(Yes at test block 4402), then block 4403 dispatches all N readyrequests.

If the number of ready requests N is greater than the number of databuffering M currently available (No at test block 4402), test block 4404determines whether the amount of data buffering M currently available isless than the hysteresis R. If the amount of data buffering M currentlyavailable is less than the hysteresis R (Yes at test block 4404), thenblock 4405 enters a wait state for one memory cycle. Flow then returnsto test block 4404 to determine whether the amount of data buffering Mcurrently available is less than the hysteresis R. In the streamingengine embodiment the amount of data buffering M depends upon the numberof unallocated cache lines. As detailed above, cache lines in datastorage 2716/2725 are deallocated when central processing unit 110processes a read and advance instruction coding. In other embodimentsthe amount of data buffering would generally be reduced upon consumptionof the data fetched. Process 4400 remains in this loop until the amountof data buffering M is greater than or equal to the hysteresis R.

If the amount of data buffering M currently available is greater than orequal to the hysteresis R (No at test block 4404), then block 4406dispatches R requests. As noted above in the streaming engine embodimentthis allows process 3400 to advance beyond test block 3401 in fetchingthe data stream.

The hysteresis of this aspect of the invention requires that no requestsare sent until a cluster of R requests can be sent together. Thisclusters R requests together whenever there are more than R requestsready. This forced clustering potentially increases the latency on anyindividual command. However, this forced clustering ensures bettertemporal and spatial locality at the memory to maximize that efficiency.The memory system accessed may include further address translation ormay include memories that have reduced latency for back-to-back samepage accesses. The forced clustering makes better use of such memoryassets. The force clustering reduces the probability of needing tochange a table look entry in address translation hardware because theclustered requests more likely have spatial locality. This tends toavoid the need for address translation entry swaps. Because the latencyfor such address translation entry swaps is indeterminate and may belengthy, average latency is reduced. The more likely spatial locality ofsuch clustered requests is more likely to permit back-to-back accessesto the same memory page. This also tends to reduce average latency.

What is claimed is:
 1. A digital data processor comprising: aninstruction memory storing instructions, wherein each of theinstructions specifies a data processing operation and at least oneoperand; a central processing unit core including: an instructiondecoder connected to the instruction memory to sequentially recallinstructions from the instruction memory and determine, for eachrecalled instruction, the specified data processing operation and the atleast one operand; and at least one functional unit connected to a dataregister file and the instruction decoder to perform a data processingoperation upon the at least one operand corresponding to an instructiondecoded by the instruction decoder and store a result of the dataprocessing operation in the data register file; and a streaming engineconnected to the instruction decoder to, in response to a stream startinstruction, recall a stream of an instruction specified sequence of aplurality of data elements from a memory, wherein the streaming engineincludes: an address generator to generate stream memory addressescorresponding to the stream of the instruction specified sequence of theplurality of data elements; a stream buffer to receive and storing datacorresponding to the stream memory addresses, wherein the stream bufferfurther includes a fault address register; and a stream head register tostore a next sequential data element of the stream of the instructionspecified sequence of the plurality of data elements to be used by theat least one functional unit, wherein the at least one functional unitis responsive to a stream operand instruction to receive at least oneoperand from the stream head register; wherein the streaming engine isconfigured to: when a fault occurs while recalling the plurality of dataelements from the memory, identify the data element that triggered thefault; signal the fault to the central processing unit core only whenthe identified data element is provided to the at least one functionalunit as an operand; and store the address of the identified data elementin the fault address register.
 2. The digital data processor of claim 1,wherein: the fault address register comprises an extended controlregister.
 3. The digital data processor of claim 2, wherein: the faultaddress register is accessible only via a debugger.
 4. The digital dataprocessor of claim 1, wherein: the instruction specified sequence of theplurality of data elements includes an instruction specified number ofdata elements having an instruction specified data size.
 5. The digitaldata processor of claim 1, wherein: the stream head register is dividedinto lanes, each lane corresponding to a size of the data elements; andthe streaming engine is further configured to store one data element ofthe stream of the instruction specified sequence of the plurality ofdata elements in each lane of the stream head register.
 6. The digitaldata processor of claim 5, wherein: if there are fewer remaining dataelements than lanes, the streaming engine stores all 0's in excesslanes.
 7. The digital data processor of claim 1, wherein: the dataelements of the instruction specified sequence of the plurality of dataelements each have the same data size.
 8. The digital data processor ofclaim 1, wherein: the data elements of the instruction specifiedsequence of the plurality of data elements each have an instructionspecified data type.
 9. The digital data processor of claim 1, whereinthe streaming engine is responsive to a stream end instruction to stoprecall of data from the memory.
 10. The digital data processor of claim1, wherein the streaming engine is configured to, upon recall of alldata elements in the instruction specified sequence of the plurality ofdata elements, stop recall of data from the memory.
 11. The digital dataprocessor of claim 1, further comprising: a level one data cacheconnected to the at least one functional unit to store data formanipulation by the at least one functional unit, the level one datacache servicing the at least one functional unit memory reads and writesif corresponding data is stored therein (cache hit), otherwise (cachemiss) referring the at least one functional unit memory reads and writesto a memory level higher than the level one data cache; and a level twodata cache connected to the level one data cache and to the streamingengine to store data for manipulation by the at least one functionalunit, the level two data cache servicing level one data cache misses onmemory reads and writes and streaming engine memory reads ifcorresponding data is stored therein (cache hit), otherwise (cache miss)referring the at least one functional unit memory reads and writes andstreaming engine reads to a memory level higher than the level two datacache.
 12. A digital data processor comprising: an instruction memorystoring instructions, wherein each of the instructions specifies a dataprocessing operation and at least one operand; a central processing unitcore including: an instruction decoder connected to the instructionmemory to sequentially recall instructions from the instruction memoryand determine, for each recalled instruction, the specified dataprocessing operation and the at least one operand; and at least onefunctional unit connected to a data register file and the instructiondecoder to perform a data processing operation upon the at least oneoperand corresponding to an instruction decoded by the instructiondecoder and store a result of the data processing operation in the dataregister file; and a streaming engine connected to the instructiondecoder to, in response to a stream start instruction, recall a streamof an instruction specified sequence of a plurality of data elementsfrom a memory, wherein the streaming engine includes: an addressgenerator to generate stream memory addresses corresponding to thestream of the instruction specified sequence of the plurality of dataelements; a stream buffer to receive and storing data corresponding tothe stream memory addresses; a stream head register to store a nextsequential data element of the stream of the instruction specifiedsequence of the plurality of data elements to be used by the at leastone functional unit, wherein the at least one functional unit isresponsive to a stream operand instruction to receive at least oneoperand from the stream head register; and a fault source register;wherein the streaming engine is configured to: when a fault occurs whilerecalling the plurality of data elements from the memory, identify thedata element that triggered the fault; signal the fault to the centralprocessing unit core only when the identified data element is providedto the at least one functional unit as an operand; and store dataidentifying the fault in the fault source register.
 13. The digital dataprocessor of claim 12, wherein: the fault source register comprises anextended control register.
 14. The digital data processor of claim 13,wherein: the fault source register is accessible only via a debugger.15. A digital data processor comprising: an instruction memory storinginstructions, wherein each of the instructions specifies a dataprocessing operation and at least one operand; a central processing unitcore including: an instruction decoder connected to the instructionmemory to sequentially recall instructions from the instruction memoryand determine, for each recalled instruction, the specified dataprocessing operation and the at least one operand; and at least onefunctional unit connected to a data register file and the instructiondecoder to perform a data processing operation upon the at least oneoperand corresponding to an instruction decoded by the instructiondecoder and store a result of the data processing operation in the dataregister file; and a streaming engine connected to the instructiondecoder to, in response to a stream start instruction, recall a streamof an instruction specified sequence of a plurality of data elementsfrom a memory, wherein the streaming engine includes: an addressgenerator to generate stream memory addresses corresponding to thestream of the instruction specified sequence of the plurality of dataelements; a stream buffer to receive and storing data corresponding tothe stream memory addresses, wherein the stream buffer includes aplurality of cache lines, each cache line including a plurality of databits, a plurality of address tag bits corresponding to the data bits andat least one valid bit indicating whether the data bits of the cacheline are valid; and a stream head register to store a next sequentialdata element of the stream of the instruction specified sequence of theplurality of data elements to be used by the at least one functionalunit, wherein the at least one functional unit is responsive to a streamoperand instruction to receive at least one operand from the stream headregister; wherein the streaming engine is configured to: when a faultoccurs while recalling the plurality of data elements from the memory,identify the data element that triggered the fault; signal the fault tothe central processing unit core only when the identified data elementis provided to the at least one functional unit as an operand; upon eachgeneration of a new address by the address generator to recall a nextdata element in the data stream: compare the next data element addressto the tag bits of each of the cache lines; and only if the next dataelement address does not match the tag bits of any of the cache lineshaving the at least one valid bit indicating data corresponding to thenext data element address is valid: supply the next data element addressto the memory to recall a corresponding data element; allocate a cacheline to the next data element; store data corresponding to the next dataaddress in the tag bit of the allocated cache line; store an indicationof valid data in the at least one valid bit of the allocated cache line;and upon return of the data corresponding to the next data elementaddress, store the data in the data bits of the allocated cache line.16. The digital data processor of claim 15, wherein: the streamingengine is further configured to: allocate a cache line of the streambuffer to the next data element only from unallocated cache lines of thestream buffer.
 17. The digital data processor of claim 16, wherein: thestreaming engine is further configured to: deallocate a cache line ofthe stream buffer following supply of corresponding data elements to theat least one functional unit.
 18. A digital data processor comprising:an instruction memory storing instructions, wherein each of theinstructions specifies a data processing operation and at least oneoperand; a central processing unit core including: an instructiondecoder connected to the instruction memory to sequentially recallinstructions from the instruction memory and determine, for eachrecalled instruction, the specified data processing operation and the atleast one operand; and at least one functional unit connected to a dataregister file and the instruction decoder to perform a data processingoperation upon the at least one operand corresponding to an instructiondecoded by the instruction decoder and store a result of the dataprocessing operation in the data register file; and a streaming engineconnected to the instruction decoder to, in response to a stream startinstruction, recall a stream of an instruction specified sequence of aplurality of data elements from a memory, wherein the streaming engineincludes: an address generator to generate stream memory addressescorresponding to the stream of the instruction specified sequence of theplurality of data elements; a stream buffer to receive and storing datacorresponding to the stream memory addresses; and a stream head registerto store a next sequential data element of the stream of the instructionspecified sequence of the plurality of data elements to be used by theat least one functional unit, wherein the at least one functional unitis responsive to a stream operand instruction to receive at least oneoperand from the stream head register; wherein the streaming engine isconfigured to: when a fault occurs while recalling the plurality of dataelements from the memory, identify the data element that triggered thefault; and signal the fault to the central processing unit core onlywhen the identified data element is provided to the at least onefunctional unit as an operand; wherein the stream operand instructionincludes a stream operand read only instruction and a stream operandread and increment instruction; wherein the at least one functional unitreceives as an operand data stored in the stream head register inresponse to the stream operand read only instruction and the streamoperand read and increment instruction; and wherein the streaming engineis further configured to store a next fetched data element in the streamhead register in response to the stream operand read and incrementinstruction and deallocate a corresponding cache line of the streambuffer upon supply of all data elements stored therein.
 19. The digitaldata processor of claim 18, wherein the data register file includes aplurality of data registers to store data, each data register designatedby a register number; and wherein the instruction decoder is configuredto: decode an instruction having a data operand field having a firstsubset of bit codings to supply data stored in a corresponding dataregister to a corresponding functional unit; decode an instructionhaving a data operand field having a predetermined read bit coding asthe stream operand read only instruction; and decode an instructionhaving a data operand field having a predetermined read/advance bitcoding as the stream operand read and increment instruction.
 20. Adigital data processor comprising: an instruction memory storinginstructions, wherein each of the instructions specifies a dataprocessing operation and at least one operand; a central processing unitcore including: an instruction decoder connected to the instructionmemory to sequentially recall instructions from the instruction memoryand determine, for each recalled instruction, the specified dataprocessing operation and the at least one operand; and at least onefunctional unit connected to a data register file and the instructiondecoder to perform a data processing operation upon the at least oneoperand corresponding to an instruction decoded by the instructiondecoder and store a result of the data processing operation in the dataregister file; a streaming engine connected to the instruction decoderto, in response to a stream start instruction, recall a stream of aninstruction specified sequence of a plurality of data elements from amemory, wherein the streaming engine includes: an address generator togenerate stream memory addresses corresponding to the stream of theinstruction specified sequence of the plurality of data elements; astream buffer to receive and storing data corresponding to the streammemory addresses; and a stream head register to store a next sequentialdata element of the stream of the instruction specified sequence of theplurality of data elements to be used by the at least one functionalunit, wherein the at least one functional unit is responsive to a streamoperand instruction to receive at least one operand from the stream headregister; wherein the streaming engine is configured to: when a faultoccurs while recalling the plurality of data elements from the memory,identify the data element that triggered the fault; and signal the faultto the central processing unit core only when the identified dataelement is provided to the at least one functional unit as an operand;and a data coherence unit connected to the streaming engine and to thememory to assure that the stream of the instruction specified sequenceof the plurality of data elements is coherent with all writes to thememory before the stream start instruction and that the stream iscoherent with all writes to the memory after the stream startinstruction.
 21. The digital data processor of claim 20, wherein thestreaming engine is responsive to a stream end instruction to stoprecall of data from the memory; and wherein the instruction memoryincludes instructions to control the at least one functional unit toavoid memory writes to memory addresses within a block of the memorythat stores the instruction specified plurality of data elements of thestream during an interval between a corresponding stream startinstruction and a corresponding stream end instruction.